Climate Change: Humanity at a Crossroads

The next few years are probably the most important in (human) history.

Dr. Debra Roberts, IPCC Working Group II Co-Chair

It is hard to know where to start when writing about climate change, as it affects nearly every aspect of human life on this planet, and at every scale. It affects our security, our food and water systems, our energy and economic systems, our infrastructure. It affects the intensity and cost of natural disasters. It affects most any ecosystem you care to study, whether terrestrial or marine. It affects our mental health, the priorities of our communities and cities, the futures of our children. It affects our politics, the openness of our societies, how we as nations relate to other nations, and how we as individuals relate to each other, especially those we see as not part of our tribe.

At only 1.2°C of mean global surface warming, many of these aspects of climate change have already become not just visible, but catastrophic. And it is crucial to realize that every climate impact will intensify as warming proceeds. In other words, what we are experiencing today is far from a “new normal”: impacts will get worse, and worse, and worse as warming proceeds. Indeed, the phrase “climate change” may no longer adequately capture the urgency of this reality, which might better be described as “climate breakdown”; and many climate scientists, myself included, are now embracing the label “alarmist.” For there is an alarm to sound.

Climate breakdown is a devilish problem for humanity. During the nineteenth century, a handful of scientists worked out how fossil fuel emissions were warming the planet. These early climate scientists were not alarmed about something so comfortably in the future; some even thought that a warmer Earth would be a good thing. With each passing decade, fossil fuel became more deeply entwined into the economy and into daily life, from transportation to construction to manufacturing to food. The infusion of fossil fuel drove spectacular increases in wealth, mobility, consumption, technology, and medicine. It powered a Green Revolution in agriculture, and the exponential growth in food inevitably drove an exponential growth in population. Human civilization today literally runs on fossil fuel. We are addicted to the stuff: try to imagine a world without it. Try to imagine a military voluntarily giving it up. As individuals, communities, corporations, and nations, we are deeply attached to the convenient, profitable, powerfulstuff.

The phrase “climate change” may no longer adequately capture the urgency of this reality, which might better be described as “climate breakdown”; and many climate scientists, myself included, are now embracing the label “alarmist.” For there is an alarm to sound

But the devil will have its due. By the 1960s, scientists began to realize the dangerous implications of global warming and to sound the alarm. A 1965 White House report, for example, warned that continued fossil fuel use would lead to “apocalyptic” and “irreversible climate change,” including just over three meters (ten feet) of sea-level rise. Despite these clear warnings, the US government responded by doing nothing. The fossil fuel corporations at first made good-faith attempts to understand the problem. But in the 1980s, sensing imminent climate action that could threaten their massive profits, they literally chose to sell out the entire world, embarking on a systematic misinformation campaign to sow confusion among the general public and delay action for as long as possible. Meanwhile, the public lacked even the usual, visible indications of pollution, such as garbage lying on the ground or smog in the air. Burning fossil fuel emits carbon dioxide (CO₂), a gas that naturally occurs in the atmosphere and traps outgoing infrared radiation, warming the Earth. Because it is odorless and colorless, CO₂ emissions fly below the radar of our awareness. And due to its incremental and global nature, it is easy to think of climate change as something safely far away or in the future. Even environmental activists were largely unaware of the climate threat until at least the 1980s.

This lack of immediacy is perhaps the biggest block to climate action, as the public continues to blithely ignore climate change at the voting booth, placing it at the bottom of their list of issues. However, even as our lives proceed apace and our day-to-day experience seems normal—although there are hints for the observant, even in our own backyards, of the massive changes underway—there is now a growing consensus among Earth scientists that climate change poses an existential threat to human civilization.

In other words, civilization is at a crossroads. Humans have become the dominant perturbation to the natural world at a global scale, mainly via two mechanisms: by expanding into and transforming the habitats of other species, and by emitting greenhouse gases and warming the planet.

Lack of immediacy is perhaps the biggest block to climate action, as the public continues to blithely ignore climate change at the voting booth, placing it at the bottom of their list of issues

Today, humanity must choose to transition its entire energy infrastructure away from fossil fuel in an incredibly short span of time—within the next thirty years—or suffer increasingly catastrophic consequences.

This brief overview of the potential of climate breakdown to disrupt humanity’s progress is no doubt a sobering counterweight to the exuberance on display elsewhere in this volume. While by some metrics humanity is doing better than ever, the overall picture is somewhat bleaker, as climate breakdown has the potential to derail all this progress. To minimize this risk, humanity must address it with the utmost urgency, with an energy of mobilization and level of cooperation never before seen in the history of our species. And unfortunately, humanity as a whole is so far showing little of this necessary urgency; indeed, in recent years there has even been some regress, as populist, anti-science regimes gain power in the United States, Europe, and elsewhere. The challenge we face as a species is indeed sobering. While those who claim that humanity is doing better than ever are bound to be the more popular messengers, perhaps this is not the time to party, but instead to get to work.

The rest of this article is organized as follows. I first look retrospectively at the decade before 2018, sampling a few climate milestones. I then discuss a few of the climate impacts projected to occur at just 1.5°C of warming and 2°C of warming. Next, I discuss some potential solutions. I argue that we need myriad solutions at all scales, an “all of the above” approach, but that we must avoid attractive but dangerous pseudo-solutions that may not turn out to be solutions at all. Finally, I zoom far out to consider our climate conundrum from an astronomical perspective, concluding with thoughts about what a “new enlightenment” might mean in the context of climate breakdown.

Because writing such a broad overview requires me both to wander outside my areas of scientific expertise and to offer my opinions, I write this paper primarily from my role as a citizen.

The Last Ten Years in Climate Change

The decade prior to 2018 saw increasing greenhouse gas emissions and warming, advances in climate science, an increase in climate-change-related disasters and costs, and an increasing sense of urgency and action.

Perhaps a good way to kick off a brief (and necessarily incomplete) overview of the last ten years in climate change is with the following simple but astounding facts: Antarctica is melting three times faster now than it was just a decade ago.¹ This is the nature of ice in a warming world.

Warming over the period 2008 to 2017 is clearly apparent in a variety of independent heat indicators. That these signals can emerge from Earth system variability over such a short period is a remarkable testament to the unprecedented rapidity of warming

The principal driver of warming is human CO₂ emissions from fossil fuel burning and deforestation. In June 2018, the atmospheric CO₂ fraction measured from Mauna Loa in Hawaii was 411 ppm, up from 388 ppm in June 2008.2 It has been increasing at a steady exponential rate of 2.2% per year since about 1790 and a preindustrial level of 280 ppm.³ Recent year-over-year increases have averaged about 2 ppm per year. This stubborn exponential increase in atmospheric CO₂ is indicative of just how foundational fossil fuel has been to human civilization since the Industrial Revolution; stopping that growth in a controlled way (as opposed to collapse) will require an energy revolution. Humanity, for its entire existence, has obtained energy primarily by burning things. Our task now is to switch directly to the Sun.

A Warming World

Warming over the period 2008 to 2017 is clearly apparent in a variety of independent heat indicators. That these signals can emerge from Earth system variability over such a short period is a remarkable testament to the unprecedented rapidity of warming. At no time in Earth’s history has it warmed this rapidly.

Figure 1 shows the global mean surface temperature since 1850, relative to the mean value of 1850–1900, from the Berkeley Earth dataset which blends sea surface temperatures over ocean and near-surface air temperatures over land.⁴ The 2015–17 mean value was 1.2°C above the baseline; the 2017 value estimated from a thirty-one-year extrapolated mean is 1.15°C above the baseline.⁵ The global mean surface warming has recently increased by 0.20°C per decade (as determined from a linear fit from 1988 to 2017). The recent decade included seven of the ten hottest years on record. If global temperature continues rising at this rate, it implies attainment of 1.5°C above the baseline in about 2035, and attainment of 2°C above the baseline in about 2060.

Figure 1. Global average surface temperature relative to the 1850–1900 mean through 2017, from Berkeley Earth. The decade 2008–17 is shown in red

Figure 2. Global average ocean heat content relative to the 1955–2006 mean. Shading gives the 90% confidence interval

Ocean heat content is an even more reliable indicator of global warming, as over 90% of the Earth’s current heat imbalance is pouring into the oceans. Figure 2 shows ocean heat content since 1955 in the layer from the surface down to a depth of two kilometers.⁶ Both global mean surface temperature and ocean heat content are increasing so rapidly that it is now becoming pointless to say, for example, that 2017 broke records for ocean heat.

Summer Arctic sea ice volume fluctuates significantly from one year to the next due to a variety of factors; it is connected to the rest of the Earth system. However, it decreased by over 20% from a mean of 18,900 km³ over the decade 1998–2007 to a mean of 14,700 km³ over the decade 2008–17.7 2017 saw record low Arctic sea ice volume of 12,900 km³. In 2017, a tanker carrying natural gas became the first merchant ship to cross the Arctic without an ice breaker; and the summer of 2018 saw the first container ship in history successfully traverse the northern Arctic route.⁸ In February 2018, there was a massive, long-lived warming event in the Arctic, with average daily temperatures up to 20°C above the 1958–2002 means. While anomalous and alarming, the implications are not yet clear.⁹

Meanwhile, sea-level rise, which has been monitored from space since 1993, has increased by 3–4 cm over the recent decade. Roughly two thirds of the rise is due to meltwater from ice sheets and glaciers and roughly one third is due to thermal expansion as the ocean warms. In 2017, global mean sea level was 77 mm higher than it was in 1993. It is accelerating by 0.084 mm per year. Sea-level rise varies geographically, with anomalies of up to 15 cm above and below this mean rise.¹⁰ Over the last decade, it has had a profound effect on tropical cyclone storm surges, low-lying nations, and sunny-day flooding in coastal cities, and costs are mounting.

Coral reefs were devastated by ocean heat waves between 2014 and 2017. More than 75% of Earth’s tropical reefs experienced bleaching stress during this period, and 30% experienced mortality stress levels.¹¹ Mass bleaching now returns on average every six years, faster than reefs can recover. This is projected to increase as warming progresses.

In 2017, a tanker carrying natural gas became the first merchant ship to cross the Arctic without an ice breaker; and the summer of 2018 saw the first container ship in history successfully traverse the northern Arctic route

There have been a large number of other profound global and regional Earth system changes over the last decade due to global warming: mountain glaciers have melted; extreme heat days have increased; spring has moved earlier; and drought, wildfire, and associated ecological transformation has increased in the US west and elsewhere.

Climate-Related Natural Disasters

The last ten years have seen a rise in climate-related natural disasters and associated costs. Classes of natural disasters with increasing intensity clearly related to global warming include tropical cyclones, wildfires, drought, and flooding.

Hurricanes and typhoons are becoming more frequent and more intense due to rapidly warming oceans, warmer atmospheres that hold more moisture, sea-level rise that intensifies storm surge, and slower motions due to climate-related jet stream changes. Cyclones are now tending to intensify more rapidly as well. Five of the six costliest Atlantic hurricanes have occurred in the decade 2008–17, the sixth being Hurricane Katrina in 2005.

As many regions become hotter and drier due to climate change, wildfires are worsening. In the state of California, for example, fifteen of the twenty largest fires in history have burned since 2000.¹² 2017 was the most destructive year for wildfires in California on record, and 2018 is on track to break that record.

Wild fires, drought, flooding and tropical cyclones are related to global warming. Five of the six costliest Atlantic hurricanes have occurred in the decade 2008–17, the sixth being Hurricane Katrina in 2005

While drought is challenging to define and measure and has multiple contributing factors, it has recently become clear (since 2013) that global warming is intensifying drought in some regions of the world. Climate-intensified drought can be caused by climate-induced shortfalls in precipitation as well as by higher temperatures, which evaporate soil moisture and decrease snowpack via earlier melt and a shift from snowfall to rain. Climate change is now thought to have intensified recent drought in California and the US southwest, and to have contributed to drought thought to be one factor in precipitating the Syrian civil war in 2011.¹³

The other side of drought is excess rainfall, which can cause flooding and mudslides. Indeed, large-scale teleconnections in the atmosphere (stationary waves) can interconnect both drying in the western US and flooding in South Asia,¹⁴ such as the 2017 monsoon which affected more than forty-five million people, killing over one thousand. In addition to changes in atmospheric dynamics, a warmer atmosphere holds more water, leading to heavier precipitation events.

The insurance industry is facing losses due to these enhanced risks, and has not had time to adapt. 2017 saw record losses in the industry.¹⁵

Advances in Climate Science

By 2007, climate scientists had long-since provided the world with unequivocal evidence for the most important societal response: burning fossil fuel causes warming, which is bringing catastrophic effects; so stop burning fossil fuel. In that sense, the last decade has seen no breakthroughs in climate science. Still, the scientific community has filled in many important details over this period; here are a handful of my personal favorites.

The use of climate models to probabilistically attribute individual weather events to climate change is advancing rapidly, something thought to be impossible in the early 2000s. In 2004, Stott, Stone, and Allen published the first attribution study for the 2003 deadly heatwave in Europe.¹⁶ Attribution studies are now performed for a wide variety of events. In 2013, the Intergovernmental Panel on Climate Change (IPCC) stated that attribution of individual droughts to climate change was not possible; and even a few years ago journalists would routinely state that no individual event can be attributed to climate change. Today, however, rigorous model-based attributions sometimes appear days after natural disasters, and soon such attributions will be made in real time.¹⁷ Real-time attribution could potentially help increase the public’s awareness of urgency. In general, attribution could have legal implications for corporate carbon polluters.

Remote sensing from satellites has advanced over the last decade. GOSAT (launched 2009) and OCO-2 (launched 2014) provide precision measurements of CO₂ concentrations for the entire planet, a crucial measurement for international cooperation on mitigation. Data from these carbon-monitoring satellites is also crucial for improving our understanding of the global carbon cycle. There has also been a quiet revolution in the remote sensing of ecological systems; for example, GOSAT and OCO-2 also serendipitously provide measurements of solar-induced fluorescence, which allows researchers to deduce plant health, stress, and productivity. In general, for many space-borne measurements which began in the 1990s, data records now extend for more than two decades, making them increasingly useful in climatological contexts.

The use of climate models to probabilistically attribute individual weather events to climate change is advancing rapidly, something thought to be impossible in the early 2000s. Stott, Stone, and Allen published the first attribution study for the 2003 deadly heatwave in Europe

Whereas in situ measurements of the atmosphere have long relied mainly on mature technologies, such as weather balloons, radar, and LIDAR, the last decade has seen a revolution in in situ ocean measurements with the advent of Argo floats. Argo is a system of about 4,000 floats distributed over the global ocean. The floats measure temperature, salinity, and current, spending most of their time drifting at a depth of one kilometer. Every ten days they descend to two kilometers and then ascend to the surface, where they relay their measurements to satellites. Argo is an international collaboration that went operational with 3,000 floats in 2007, and since then has revolutionized our ability to measure the Earth’s energy imbalance.

Climate models have also steadily improved. In 2007, the IPCC’s AR4 was released and global climate models (from the third Coupled Model Intercomparison Project, CMIP3) had typical horizontal resolutions of about 110 kilometers; resolution has since improved, and adaptive mesh refinement places high resolution where it is most needed. In 2008, models were used for the first time to study climate tipping points, such as Arctic sea ice and the Greenland ice sheet. Modeling of aerosols were improved, and black carbon is given more attention.¹⁸ CMIP5 served as a foundation for the IPCC AR5, which was released in 2013 and included an in-depth evaluation of the CMIP5 model ensemble. Today, as we head into CMIP6 and IPCC AR6, most projections of global change will come from Earth System Models (ESMs), which include coupled ecosystem and biosphere models in addition to atmosphere, ocean, ice, and land. The trend is toward increasing spatial resolution, as computer equipment continues to become more powerful. Advances in regional modeling and downscaling have also allowed for increasingly useful regional projections.

In summary, we can now monitor, measure, and model the Earth system with more precision and accuracy than ever. However, there is still much room for improvement. For example, the estimated range of equilibrium climate sensitivity to a doubling of CO₂has been essentially unchanged at 1.5°C to 4.5°C since the Charney report in 1979. Difficult modeling challenges remain, such as improving the representation of clouds and aerosols, carbon cycle feedbacks, and vegetation; and capturing nonlinearities’ tipping points. All models are wrong, but some are useful.¹⁹ The modeling and observational communities have been working together to make models—the bridge to knowledge about the future—more useful.

In 2008, models were used for the first time to study climate tipping points, such as Arctic sea ice and the Greenland ice sheet. Modeling of aerosols were improved, and black carbon is given more attention

Finally, it is worth mentioning that over the last decade it has been found that essentially all working climate scientists (at least 97%) agree that humans are warming the planet.²⁰ But what about those 3% who deny this consensus? They amounted to thirty-eight peer-reviewed papers over the last decade. It turns out that every single one of them had errors, and when the errors were corrected, the revised results agreed with the consensus in every case.²¹

Humanity’s Response

The most important metric for assessing humanity’s response over the last decade is CO₂ emissions, and, globally, emissions are still increasing. The unavoidable conclusion is that whatever humanity is doing, it is not working; at least not yet.

In 2017, the four largest emitters were China (28%), the US (15%), the EU (10%), and India (7%).²² China’s emissions began increasing dramatically around 2000 and surpassed the US in 2005. However, China’s emissions decreased by 0.3% from 2015 to 2017, while India’s emissions increased 4.5% over the same period. India and China have the same population (1.3 and 1.4 billion people, respectively). While India has had a relatively minor part in warming over the recent decade, it will become increasingly important going forward.

Figure 3. CO2 emissions from fossil fuel and industry. Global emissions from fossil fuels and industry were projected to rise by 2% in 2017, stubbornly consistent with historical exponential emissions growth. Source: Global Carbon Project

In 2013, the nations of the world met in Paris to discuss climate action under the United Nations Framework Convention on Climate Change. Given how little had been accomplished previously on the international stage, even orchestrating this meeting was a significant achievement. If the Paris Agreement were to be honored, it would lead to global greenhouse gas emissions (CO₂and non-CO₂) in 2030 of about 55 GtCO₂-eqivalents per year, which is not nearly enough mitigation to keep warming under 1.5°C and would lead to warming well in excess of 3°C.²³ Essentially all available 1.5°C pathways have emissions below 35 GtCO₂eq/yr in 2030, and most require even less. The plan was to strengthen the Paris Agreement going forward in order to meet targets. However, the United States and President Trump loudly proclaimed that it will not only ignore the agreement, but will actively pursue policies to increase emissions, such as coddling the coal industry. With the world’s largest contributor to climate change in terms of accumulated emissions out, other countries might leave as well.

With a vacuum at the federal level in the US and other countries, cities and states are attempting to take mitigation into their own hands. C40 cities is a network of the world’s megacities committed to addressing climate change, founded in 2005. Member cities must have a plan to deliver their contribution to constraining warming to no more than 1.5°C. My home state of California, the world’s fifth largest economy, is a leader within the US on climate action. In 2018, California passed groundbreaking legislation to get to 60% renewable electricity by 2030 and 100% carbon-free electricity by 2045.

In 2017, the four largest emitters were China (28%), the US (15%), the EU (10%), and India (7%). China’s emissions began increasing dramatically around 2000 and surpassed the US in 2005. However, China’s emissions decreased by 0.3% from 2015 to 2017

Renewables have been growing globally, driven largely by cost decreases and volume from China. In 2016, solar photovoltaic capacity grew by 50%; and in 2017 solar added 98 gigawatts globally, more than fossil fuel and nuclear combined. China now has more than 3 million jobs in solar, over 1,000 times the number of solar jobs in the US (230,000).²⁴ While the US had a chance to take the lead in the clean energy economy five or ten years ago, today China has a commanding lead. In 2017, just under 5% of the world’s electricity was generated by solar and wind.²⁵

China is also taking the lead in electric vehicles (EVs). Total EVs in China in 2017 numbered 3.1 million, up 56% from 2016.²⁶ In Norway in 2017, 39% of car sales were EVs. Ireland has proclaimed that it will not allow sales of internal combustion vehicles after 2030. In California, EVs accounted for almost 8% of light-vehicle sales in April of 2018.

While it is hard to measure cultural shift, I feel that the last decade has seen a significant cultural shift on climate. Grassroots movements and direct action are gaining momentum. The Keystone XL fight and Standing Rock were lines drawn in the sand against fossil fuel extraction; these and other actions are raising awareness that fossil fuel is harmful. In Europe, there is a major movement against airport expansion. Academics are attempting to shift their culture of frequently flying in airplanes. The media still usually fails to mention climate change after climate-related disasters, but this may be beginning to change. The climate litigation movement is beginning to gain traction, with youth plaintiffs suing the US government in what might turn out to be a watershed case. The Sierra club has stopped over two hundred new coal plants and secured 275 existing plants for retirement. My anecdotal sense is that people in the US are talking about climate change in their everyday lives more than they did in 2008, but still not enough.

The popular climate movement is transforming into the climate justice movement, a response to the deep injustice of climate breakdown: those who contributed to it the most (wealthy people in wealthy nations in the Global North) will suffer the least, and those who contributed the least (poor people in poor nations in the Global South) will suffer the most. Half of global emissions are produced by 10% of the global population. Climate justice could help move us to an inclusive “all of the above” mindset more quickly, creating the “movement of movements” that is needed. The climate justice movement must never lose sight of the most important metric: emissions. Greenhouse gas molecules do not care about our politics, and a powerful movement must include everyone. A principal focus on reducing emissions could help minimize the risk of further politicization of climate action.

In 2016, solar photovoltaic capacity grew by 50%; and in 2017 solar added 98 gigawatts globally, more than fossil fuel and nuclear combined

In summary, the human response in all its inevitable complexity has begun to ramp up at every scale. It is currently insufficient, but a cultural shift is occurring. People are inspiring each other.

One recent night, a friend called to tell me that a nearby community’s city council would be voting in a couple of hours on whether to make their residents’ default electrical mix 50% renewable or 100% renewable. Here in California, many of our communities have been adopting “community choice energy” under a state law allowing cities and counties to band together and choose how their electricity mix is generated. Residents are enroled in a particular plan by default, and can opt into another plan. Since the vast majority remain in the default, our mission was to lobby the city council for the 100% renewable option. So I headed downtown. I spoke for two minutes, and the council began deliberations. My friend, running late from a similar action in another city, arrived just before the vote, and was given a chance to speak as well. The council voted 5–0 in favor of the 100% renewable default option. After the vote, three of the council members actually came down from the bench and exchanged hugs with us. It was a small action, but it felt wonderful.

Each one of us is a single mammal. We are limited in what we are able do. It is therefore natural to feel overwhelmed by climate breakdown. But shutting down will only make us feel worse. If we each do all we can, it may not be enough; but we cannot do more. Doing all we can connects us to everyone else around the world also doing all they can. It is the best cure for the climate blues.²⁷

The Carbon Budget for 1.5°C

Humanity as of 2017 is emitting 42 ± 3 GtCO₂ per year, or about 1 GtCO₂ every 8.6 days,²⁸ up from 34 GtCO₂ per year in 2007 (the 2002–12 mean).²⁹ This implies humanity’s CO₂ emissions have been increasing at about the same 2.2% annual rate as the atmospheric CO₂ fraction. Over these eleven years, the human population increased by 13%, or nearly a billion people (from 6.7 billion in 2007 to 7.6 billion in 2017). Global per capita CO₂ emissions increased by 9%. In other words, global emissions are driven by growth in both population and individual consumption.

By 2017, humanity had emitted about two trillion tonnes of CO₂ cumulatively (2200 ± 320 GtCO₂).³⁰ According to the IPCC, a 1.5°C maximum is likely still physically possible. By the end of 2018, for a two-thirds chance of staying below 1.5°C the IPCC estimates that humanity can emit an additional 530 GtCO₂ or 380 GtCO₂, depending on whether temperature is measured using sea surface temperatures or near-surface air temperatures over the ocean, respectively.³¹

At current rates of emissions, the first year the planet will surpass the 1.5°C mark would likely be in the late 2020s or early 2030s.³² If emissions continued, the planet would surpass 2°C of warming around mid-century.

If humanity were to begin ramping down immediately and achieve net-zero CO₂ emissions by 2055, and in addition began ramping down non-CO₂ forcings by 2030, CO₂ emissions would remain within the 530 GtCO₂ budget.³³ This would require global cooperation and mobilization greater even than World War II, and no fighting between nations could be afforded.

If humanity achieved net-zero CO₂ emissions and declining (or net-zero) non-CO₂ forcings on these approximate timescales, anthropogenic warming would likely halt after a few decades.³⁴ Earth system feedbacks, such as permafrost methane release, loss of forests, and the ice-albedo feedback, may continue to cause additional warming long after this point; the trajectory of that warming (its strength as a function of time, and its ultimate duration) is unknown.³⁵ A gradually decreasing fraction of accumulated atmospheric CO₂ would remain in the atmosphere for thousands of years, causing the global temperature to decline gradually as well; maintenance of anthropogenic warming on a timescale of centuries would cause additional ice loss and sea-level rise.³⁶ Carbon dioxide removal (CDR) could mitigate these long-term impacts, but the feasibility of large-scale CDR is unknown.

The relatively simple framing of CO₂ budgets hides much complexity and uncertainty, and the scientific community feels that the IPCC’s estimate is more likely overly optimistic than overly pessimistic, for two principal reasons. First, the IPCC used a preindustrial baseline of 1850–1900. Because large-scale fossil CO₂ emissions began about a hundred years earlier, this late-nineteenth-century baseline could understate global mean temperatures by up to 0.2°C.³⁷ In the worst case, this would imply, then, that humanity was in arrears by about a decade, or roughly 400 GtCO₂, leaving a budget of as little as 100 GtCO₂. Second, the IPCC has not attempted to include carbon cycle feedbacks such as permafrost melting and wetland emissions; according to the IPCC SR1.5, emissions from these feedbacks could detract 100 GtCO₂ from the 1.5°C budget. Due to these two considerations alone, therefore, it is conceivable that humanity has already burned through the entire CO₂ budget for 1.5°C. Each passing day’s emissions, of course, make “lock in” of 1.5°C more likely.

Additional sources of uncertainty in the budget include incomplete knowledge of the climate response (±400 GtCO₂) and uncertainty about how humanity will mitigate non-CO₂ forcings (methane, black carbon, nitrous oxide, and hydrofluorocarbons, ±250 GtCO₂).

It is important to remember that 1.5°C is an arbitrary threshold for communicating risks and setting goals. As for what actually matters on the planet, the warmer it gets, the worse the impacts will be. Whatever the actual (essentially unknowable) amount of CO₂ we can emit while remaining below any arbitrary level of warming, be it 1.5°C or 2°C, humanity’s course of action remains the same: reduce emissions as fast as possible (mitigation), and buckle in for a wild ride (adaptation).

What We Stand to Lose: 1.5°C, 2°C, and Beyond

It does seem clear, however, that 1.5°C is the best humanity can possibly hope for at this point, and unfortunately this level of warming will intensify impacts beyond today’s. Here I present a brief survey of impacts. Due to space constraints, I focus on comparing impacts at 1.5°C to impacts at 2°C; and although impacts are expected to exhibit deep regional variation, I focus here on the global picture.

Heat extremes over land are projected to shift to about 3°C hotter, although this shift will depend on region; generally speaking, expect a 47°C heatwave to clock in at about 50°C in the warmer world.³⁸ At 2°C, heatwaves will be 4°C hotter than they currently are; expect a 47°C heatwave to clock in at 51°C. Frequency of warm extremes (that would occur over land once in twenty years in today’s climate) are projected to increase by 129% under 1.5°C of warming, and by 343% under 2°C of warming.³⁹ Warm spell durations are projected to increase on average by seventeen days under 1.5°C of warming, and by thirty-five days under 2°C of warming.⁴⁰

The frequency of extreme rainfall over land is projected to increase by 17% under warming of 1.5°C and 36% under warming of 2°C.⁴¹

Heat extremes over land are projected to shift to about 3°C hotter, although this shift will depend on region; generally speaking, expect a 47°C heatwave to clock in at about 50°C in the warmer world

Sea level is projected to rise 26 cm to 77 cm by 2100 for 1.5°C of warming, and 2°C of warming is projected to bring an additional 10 cm; even this seemingly modest additional rise likely translates into an additional ten million displaced people globally.⁴² Annual economic losses due to foods are estimated at US$10 trillion under 1.5°C of warming, and US$12 trillion under 2°C of warming.⁴³ Sea level will continue to increase beyond 2100, with a potential for increases of several meters over hundreds of years as ice sheets are irreversibly lost.⁴⁴ However, the trajectory of ice-sheet loss and the resulting sea-level rise is still highly uncertain. Ice-sheet scientists are increasingly pointing out the potential for a much larger, much more rapid sea-level rise due to nonlinear loss of ice sheets, especially in west Antarctica, which could be triggered by global warming of even just 1.5°C.⁴⁵ Such a rapid increase would have profound implications for the world’s coastal cities, the global economy, and global political stability.

Relative to 1.5°C, average vertebrate habitat loss doubles, and average invertebrate habitat loss triples at 2°C.⁴⁶ Relative to 1.5°C, an additional 1.5 million to 2.5 million square kilometers of permafrost are projected to melt at 2°C.⁴⁷

At 1.5°C, between 70% and 90% of the world’s warm water coral reefs are projected to be lost; at 2°C, 99% of reefs would be lost.⁴⁸ At 1.5°C, the global annual fish catch is projected to decrease by 1.5 million tonnes compared to 3 million tonnes for 2°C.⁴⁹

The Atlantic meridional overturning circulation is projected to decrease by 11% under 1.5°C of warming and by 34% under 2°C of warming in 2100.⁵⁰

Biodiversity losses are rapid and long-lasting; for example, biodiversity loss in mammals expected to occur over the next fifty years will last for millions of years, as evolution gradually recreates diversity.⁵¹ I personally find it staggering that the consequences of the decisions we make over the next few decades will stretch out for millions of years.

Health risks increase with temperature. The suitability of drylands for malaria transmission is projected to increase by 19% under 1.5°C of warming and 27% under 2°C of warming.⁵² As temperatures increase, there will be more numerous Aedes mosquitoes, and over a larger geographic range, increasing the risk of dengue fever, chikungunya, yellow fever, and Zika virus. The range and seasonality of Lyme and other tick-borne diseases are projected to expand in North America and Europe, and projections worsen with temperature.⁵³ At 1.5°C, twice as many megacities are likely to experience heat stress, exposing 350 additional people to deadly heat by 2050.⁵⁴

One study projects global maize yields to decrease by 6% under warming of 1.5°C and 9% under warming of 2°C by 2100

The integrated global food system is complex, to say the least. Future food security will depend on the interplay between regional crop stresses (temperature, water, pests, soil depletion), adaptation, population, consumption patterns, energy costs, and international markets. So far, technological yield gains have managed to keep pace with global warming, but this may not last as stresses increase. One study projects global maize yields to decrease by 6% under warming of 1.5°C and 9% under warming of 2°C by 2100.⁵⁵ Another study projects that mean yields in four countries responsible for over two thirds of maize production (the US, Brazil, Argentina, and China) will decrease by 8–18% under 2°C of warming, and by 19–46% under 4°C of warming.⁵⁶ (These estimates do not include additional stresses from aquifer depletion.)

It is possible that yields of all crops could drop more steeply under higher temperatures, as plant and pest responses to warming are not expected to remain linear; this steep drop with temperature ensures that warming will bring greater harvest variability. With increasing likelihood of simultaneous yield losses in multiple regions and multiple crops, serious price shocks such as the tripling that occurred in 1972–1974 (due to extreme temperatures in the USSR) could become frequent.⁵⁷ When projected yield losses and variability increases are combined with projected population growth, it is a cause for concern.

Global poverty is projected to increase with warming. At 1.5°C, the number of people exposed to climate-related poverty is reduced by up to several hundred million by 2050 relative to 2°C.⁵⁸

Beyond 2°C of warming, impacts get much worse and adaptation gets more expensive and less effective.

Near-Term Solutions: “All of the Above” but “Keep It Real”

Preventing more than 1.5°C of warming above the 1850–1900 baseline at this late date (if it is still possible) would require a massive global mobilization, far exceeding current levels of action and even current pledges of the Paris Agreement. In my opinion, to succeed, humanity will need to make climate action its highest priority, above even economic growth.

The recent IPCC special report on global warming of 1.5°C (IPCC SR1.5) suggests that such a goal is at least still physically possible.⁵⁹ Whether this is true or not, however, is immaterial to humanity’s course of action. The path to 2°C of warming and beyond is via 1.5°C of warming, and, as we saw in the previous section, impacts worsen profoundly as warming increases.

While the fine details of these projected impacts are still the subject of scientific debate, the essential fact of dangerous anthropogenic warming is not. The human response, however, will always be up for debate. The recommendations that follow are therefore necessarily my opinions as a global citizen.

Overall, we need to be very careful to adopt solutions that will result in actual emissions reductions, here and now. Call me crazy, but I think that a good way to respond to a crisis caused by burning fossil fuel is to stop burning fossil fuel, at every scale. We should not necessarily expect life to go on exactly as it has in the age of fossil fuel. And that’s OK: a livable planet is far more important than preserving a status quo that has not even been making us happy.

Dangerous Thinking

Assessments of the utility of a specific climate action must also include its feasibility. Unfortunately, human nature gravitates to potential solutions that require no immediate change, but that appear somehow technologically sexy. Perhaps many of us tend to fetishize such techno-fixes because they resonate with one of our society’s most powerful myths: progress. Such “solutions” are dangerous because they reduce urgency and divert from meaningful climate action.

Perhaps the most dangerous example of this sort of magical thinking is negative emissions technologies (NETs). Leading NET schemes include bioenergy carbon capture and storage (BECCS) and enhanced rock weathering. While I believe that research on NET schemes should be ramped up, counting on them to solve climate breakdown is dangerously irresponsible. These technologies do not exist at scale yet, and may not exist in time to help. To assume that NETs will someday draw down CO₂ from the atmosphere, thereby magically atoning for today’s carbon, is to burden our children with both paying for the scheme (guaranteed by the second law of thermodynamics to be costly) and suffering with a higher level of warming than if mitigation had urgently proceeded.

Another example is agricultural biofuel. Given the projections for food insecurity described in the previous section, there is no feasible path for agricultural biofuel to become a major component of climate action. Furthermore, the energy return on energy invested (EROEI) for corn ethanol is approximately 1:1, meaning it takes as much energy to produce corn ethanol as is released by burning corn ethanol.60 Even inefficient tar sands oil has an EROEI of 4:1, and wind power has an EROEI of 20:1. While research should continue on unconventional energy sources such as artificial photosynthesis, these programs should not be seen as “solutions” for the simple reason that they do not yet exist, and may not exist in time to mitigate the current climate crisis. To mitigate the current climate crisis effectively, they would likely need to be deployed and scaled up to global levels within the next ten years or so (that is, before 2030); this seems exceedingly unlikely. The longer it takes for them to arrive, the less effective they will be.

Research on NET schemes should be ramped up, but counting on them to solve climate breakdown is dangerously irresponsible. These technologies do not exist at scale yet, and may not exist in time to help

Solar geoengineering refers to large-scale efforts by humanity to reflect a fraction of incoming sunlight.61 Of the possible schemes, the most popular involves mimicking volcanic eruptions by distributing bright aerosols into the stratosphere. Unlike NETs, aerosol geoengineering is technologically and economically feasible. Unfortunately, it could cause additional disruptive climate and precipitation changes; it would not address ocean acidification; and worst of all, it could set up a “termination shock,” saddling our children with a massive, sudden temperature spike if civilization becomes unable for any reason to continue supporting a large fleet of pollution-spraying airplanes. Our understanding of the ramifications of aerosol geoengineering is still crude, and research in this area should accelerate. It will be unfortunate if humanity feels that aerosol geoengineering is its best option, but this could soon be the case if procrastination continues and climate impacts are allowed to worsen. In this case it would be far better to have mature research in hand to guide the project.

Meaningful Action

Unlike the above pseudo-solutions, meaningful action will require cultural shift and broad public support. Burning fossil fuel causes harm, and must no longer be seen as acceptable. The public does not understand the urgency of the situation. To me, this is actually a source of hope: once the public does wake up, the rapidity of action could be genuinely surprising. Public support would cause powerful institutions and governments to fight for climate action, instead of against it. To me, the difference would be like night and day.

Once the public wakes up, the rapidity of action could be genuinely surprising. Public support would cause powerful institutions and governments to fight for climate action, instead of against it. To me, the difference would be like night and day

Hastening this cultural shift is up to each of us. There is no shortcut. I think this is an empowering message: personal actions matter, because individual, community, and collective action are inextricably intertwined. Collective action enables individual action (by changing systems) and individual action enables collective action (by changing social norms). The distinction between individual and collective action blurs, and it turns out to be a false dichotomy. We need action at all scales. For those who are concerned about climate breakdown, I recommend deeply and systematically reducing your own emissions, and working to spread that change within your community and beyond, via people you know, community institutions, and passionate communication aligning with your talents.⁶²

In the very near term, the best first step any nation could take would be to implement a carbon fee and dividend.⁶³ Whenever any fossil fuel is taken from the ground or imported over a border, a fee would be assessed based on the embodied CO₂ emissions (and possibly also on other greenhouse gases). The fee would start at a modest level (likely less than $100 per tonne CO₂) and increase every year at a scheduled rate, eventually becoming prohibitively high (thousands of US dollars). This would fix a glaring problem in the market—that while climate pollution is incredibly costly to civilization, it is still free to pollute. Alternatives to fossil fuel would thus become increasingly viable throughout the economy, from carbon-free electricity, to locally grown organic food, to sustainable manufacturing, to slow travel options other than flying. The price predictability would accelerate a phased, orderly divestment from fossil fuel, and systematic investment in alternatives. People, institutions, and businesses would naturally also look for ways to use energy more efficiently (including food). Diets would naturally shift away from meat, and innovative meat substitutes would continue to be developed and embraced by consumers.

One hundred percent of the revenue would then be returned to every citizen. This would ensure that poor households would not be penalized by the policy; in fact, the poorest three quarters or so of households would come out ahead, since wealthy people use more energy, and would contribute proportionally more to the revenue. The dividend would make the policy popular, and the policy ought to be amenable to both conservatives (since it fixes the market, does not grow the government, and does not hurt the economy) and liberals (since it protects the environment and helps poor households and marginal communities). Imagine that: a climate action that unites.

Another excellent step any nation could take would be to simply use less energy. This would make the energy transition much easier. For example, I estimate that halving electricity usage in the US would require building only a quarter as much new carbon-free generative capacity; it would also support electrification of other sectors, such as transportation, industry, and heating and cooling. Using less energy would require government regulation, which, of course, requires public support to do in any meaningful way. The US state of California is an excellent example in this regard, and continues to demonstrate what is actually possible.

Dealing with non-CO₂ emissions (such as hydrofluorocarbons) and deforestation would also benefit from regulation. First, nations will need to decide to lead within their own borders. They will then need to advocate for strong, enforceable international regulations. In other words, the cultural shift will need to become strong enough to support meaningful action at the international level. To do this will require lead nations such as China, the US, and EU nations to recognize their outsized role in the climate crisis, and agree to provide appropriate economic support to countries who have had much smaller roles. For example, lead nations benefit if Indonesia stops deforestation; perhaps they should provide economic support for this, to offset Indonesia’s financial losses. More generally, the cultural shift will need to become strong enough to begin atonement (recognition of responsibility) and reparations (economic support).

Lead nations such as China and the US, as well as the EU, must recognize their outsized role in the climate crisis, and agree to provide appropriate economic support to countries who have had much smaller roles. Lead nations benefit if Indonesia stops deforestation; perhaps they should provide economic support for this, to offset Indonesia’s financial losses

In a democracy, major policy programs such as a carbon fee and dividend and powerful regulations to rapidly and systematically deal with non-CO₂ emissions require policy makers who support such programs; having those policy makers in office requires them winning elections on platforms that include such programs; winning those elections requires having sufficient votes; having sufficient votes requires a public that actively supports the policies; and having that supportive public requires a cultural shift. Polls in the US, for example, consistently show climate at or near the bottom of the list of issues of concern for both Republican and Democratic voters; this needs to change.

Therefore, the question “How do we get policies needed for rapid, meaningful action on climate change” is really the same as “How do we get a cultural shift to climate concern”?

Humanity also needs to move toward respecting ecological constraints as rapidly as possible. In my opinion, meaningful collective action that leads to civilizational respect for planetary boundaries will require the cultural shift to proceed even further than required for strong regulatory frameworks and carbon fees. In my personal vision for how humanity can avoid further civilizational collapse, I envision climate regulations and carbon fees happening first, and feeding back into an even larger cultural shift by inspiring hope and improving lives. The near-term, feasible step of a strong fee and dividend, for example, could catalyze a shift in narrative—from “joyless human dominion and growth” to “joyful stewardship on a fragile and interconnected Earth.” It is difficult to imagine the transformative power such a shift could unleash.

The Astronomical Perspective

When viewing the iconic photograph known as “Earthrise,” taken by Apollo 8 astronaut Bill Anders in 1968, one cannot help but see our planet as the fragile, astronomically unlikely oasis in a dark cold void that it actually is. We live on spaceship Earth, together with each other and the rest of life as we know it. The world that many of us and our collective culture take for granted, to “have dominion over,” is in reality a tiny, isolated system perfectly situated relative to a spherical nuclear inferno and with just the right mixture of chemicals in its paper-thin atmosphere to be congenial to human life and human civilization. Of all the accomplishments in climate science over the last ten years, perhaps this is the most important: adding to the unequivocal evidence of the fragility of Earth’s climate, and that humans are now singlehandedly moving it beyond the safe zone for civilization.

Climate breakdown might help to explain the perplexing silence emanating from the billions and billions of planets orbiting almost every star in the night sky

Astrobiology is the study of how life arises on planets throughout the universe. Astrobiologists now speculate that climate breakdown may be a challenge not just for human civilization, but for civilizations on other planets, as well.⁶⁴ Climate breakdown might help to explain the perplexing silence emanating from the billions and billions of planets orbiting almost every star in the night sky. Perhaps ours is not the only civilization to discover fossil fuel and realize its usefulness, become addicted to it, and dangerously warm its planet’s climate.

It was natural that humanity made use of fossil fuel to build civilization. But now that we so clearly know that burning fossil fuel causes irreversible harm at a planetary scale, we must transition as though the future of human and nonhuman life on Earth depends on it.

Toward a New Enlightenment: Humanity’s Place in the Web of Life

As humanity goes forward, it would do well to examine why it has arrived at this civilizational crossroads, and what lessons can be learned. The two most urgent crises facing the global biosphere, and therefore humanity, are climate change and habitat loss. Both of these arise from the prevailing mindset of the dominant globalized culture of “take and consume,” with little thought of how one’s actions might affect other beings on spaceship Earth, both human and nonhuman.

The main lesson to be learned, then, might be that everything in the biosphere—including any individual human, and certainly the human species—is not separate, but connected to everything else. This logically leads to a golden rule for sustainable civilization: treat the Earth and its beings as you would have it and them treat you—because they are you. A key difference between those with the mindset of separation and those with the mindset of connection is that the former take for granted, and selfishness informs their actions; while the latter feel deep gratitude for all the factors of their existence, and this gratitude informs their actions.

In addition to a rapid transition away from fossil fuel, humanity must confront the other major ongoing global crises: habitat loss. At least half of the Earth must be designated for nonhuman habitat to make space for other species.⁶⁵ This might be the only way our planet can begin to regenerate a healthy biosphere, which is certainly in humanity’s long-term self-interest. This would require an entirely new paradigm for humanity: a paradigm not of uncontrolled exponential growth, but, instead, a collective self-awareness, respect for limits, and a joyful humility at being just one strand of the web of life on this beautiful planet.

Peter Kalmus

Notes