As the Intergovernmental Panel on Climate Change (IPCC) released its Sixth Assessment Report, summarized nicely on these pages by Bob Henson, much of the associated media coverage carried a tone of inevitable doom.
These proclamations of unavoidable adverse outcomes center around the fact that in every scenario considered by IPCC, within the next decade average global temperatures will likely breach the aspirational goal set in the Paris climate agreement of limiting global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial temperatures. The report also details a litany of extreme weather events like heatwaves, droughts, wildfires, floods, and hurricanes that will all worsen as long as global temperatures continue to rise.
While United Nations Secretary-General António Guterres rightly called the report a “code red for humanity,” tucked into it are details illustrating that if – BIG IF – top-emitting countries respond to the IPCC’s alarm bells with aggressive efforts to curb carbon pollution, the worst climate outcomes remain avoidable.
The IPCC’s future climate scenarios
In the Marvel film Avengers: Infinity War, the Dr. Strange character goes forward in time to view 14,000,605 alternate futures to see all the possible outcomes of the Avengers’ coming conflict. Lacking the fictional Time Stone used in this gambit, climate scientists instead ran hundreds of simulations of several different future carbon emissions scenarios using a variety of climate models. Like Dr. Strange, climate scientists’ goal is to determine the range of possible outcomes given different actions taken by the protagonists: in this case, various measures to decarbonize the global economy.
The scenarios considered by IPCC are called Shared Socioeconomic Pathways (SSPs). The best-case climate scenario, called SSP1, involves a global shift toward sustainable management of global resources and reduced inequity. The next scenario, SSP2, is more of a business-as-usual path with slow and uneven progress toward sustainable development goals and persisting income inequality and environmental degradation. SSP3 envisions insurgent nationalism around the world with countries focusing on their short-term domestic best interests, resulting in persistent and worsening inequality and environmental degradation. Two more scenarios, SSP4 and SSP5, consider even greater inequalities and fossil fuel extraction, but seem at odds with an international community that has agreed overwhelmingly to aim for the Paris climate targets.
The latest IPCC report’s model runs simulated two SSP1 scenarios that would achieve the Paris targets of limiting global warming to 1.5 and 2°C (2.7 and 3.6°F); one SSP2 scenario in which temperatures approach 3°C (5.4°F) in the year 2100; an SSP3 scenario with about 4°C (7.2°F) global warming by the end of the century; and one SSP5 ‘burn all the fossil fuels possible’ scenario resulting in close to 5°C (9°F), again by 2100.
The report’s SSP3-7.0 pathway (the latter number represents the eventual global energy imbalance caused by the increased greenhouse effect, in watts per square meter), is considered by many experts to be a realistic worst-case scenario, with global carbon emissions continuing to rise every year throughout the 21st century. Such an outcome would represent a complete failure of international climate negotiations and policies and would likely result in catastrophic consequences, including widespread species extinctions, food and water shortages, and disastrous extreme weather events.
Scenario SSP2-4.5 is more consistent with government climate policies that are currently in place. It envisions global carbon emissions increasing another 10% over the next decade before reaching a plateau that’s maintained until carbon pollution slowly begins to decline starting in the 2050s. Global carbon emissions approach but do not reach zero by the end of the century. Even in this unambitious scenario, the very worst climate change impacts might be averted, although the resulting climate impacts would be severe.
Most encouragingly, the report’s two SSP1 scenarios illustrate that the Paris targets remain within reach. To stay below the main Paris target of 2°C (3.6°F) warming, global carbon emissions in SSP1-2.6 plateau essentially immediately and begin to decline after 2025 at a modest rate of about 2% per year for the first decade, then accelerating to around 3% per year the next decade, and continuing along a path of consistent year-to-year carbon pollution cuts before reaching zero around 2075. The IPCC concluded that once global carbon emissions reach zero, temperatures will stop rising. Toward the end of the century, emissions in SSP1-2.6 move into negative territory as the IPCC envisions that efforts to remove carbon from the atmosphere via natural and technological methods (like sequestering carbon in agricultural soils and scrubbing it from the atmosphere through direct air capture) outpace overall fossil fuel emissions.
Meeting the aspirational Paris goal of limiting global warming to 1.5°C (2.7°F) in SSP1-1.9 would be extremely challenging, given that global temperatures are expected to breach this level within about a decade. This scenario similarly envisions that global carbon emissions peak immediately and that they decline much faster than in SSP1-2.6, at a rate of about 6% per year from 2025 to 2035 and 9% per year over the following decade, reaching net zero by around the year 2055 and becoming net negative afterwards.
For perspective, global carbon emissions fell by about 6-7% in 2020 as a result of restrictions associated with the COVID-19 pandemic and are expected to rebound by a similar amount in 2021. As IPCC report contributor Zeke Hausfather noted, this scenario also relies on large-scale carbon sequestration technologies that currently do not exist, without which global emissions would have to reach zero a decade sooner.
More warming means more risk
The new IPCC report details that, depending on the region, climate change has already worsened extreme heat, drought, fires, floods, and hurricanes, and those will only become more damaging and destructive as temperatures continue to rise. The IPCC’s 2018 “1.5°C Report” had entailed the differences in climate consequences in a 2°C vs. 1.5°C world, as summarized at this site by Bruce Lieberman.
Consider that in the current climate of just over 1°C (2°F) warmer than pre-industrial temperatures, 40 countries this summer alone have experienced extreme flooding, including more than a year’s worth of rain falling within 24 hours in Zhengzhou, China. Many regions have also experienced extreme heat, including the deadly Pacific Northwest heatwave and dangerously hot conditions during the Olympics in Tokyo. Siberia, Greece, Italy, and the US west coast are experiencing explosive wildfires, including the “truly frightening fire behavior” of the Dixie fire, which broke the record as the largest single wildfire on record in California. The IPCC report warned of “compound events” like heat exacerbating drought, which in turn fuels more dangerous wildfires, as is happening in California.
The IPCC report notes that the low-emissions SSP1 scenarios “would lead to substantially smaller changes” in these sorts of climate impact drivers than the higher-emissions scenarios. It also points out that with the world currently at around 1°C of warming, the intensity of extreme weather will be twice as bad compared to today’s conditions if temperatures reach 2°C (1°C hotter than today) than if the warming is limited to 1.5°C (0.5°C hotter than today), and quadruple as bad if global warming reaches 3°C (2°C hotter than today). For example, what was an extreme once-in-50-years heat wave in the late-1800s now occurs once per decade, which would rise to almost twice per decade at 1.5°C, and nearly three times per decade at 2°C global warming.
Climate’s fate has yet to be written
At the same time, there is no tipping point temperature at which it becomes “too late” to curb climate change and its damaging consequences. Every additional bit of global warming above current temperatures will result in increased risks of worsening extreme weather of the sorts currently being experienced around the world. Achieving the aspirational 1.5°C Paris target may be politically infeasible, but most countries (137 total) have either committed to or are in the process of setting a target for net zero emissions by 2050 (including the United States) or 2060 (including China).
That makes the SSP1 scenarios and limiting global warming to less than 2°C a distinct possibility, depending on how successful countries are at following through with decarbonization plans over the coming three decades. And with its proposed infrastructure bipartisan and budget reconciliation legislative plans – for which final enactment of each remains another big IF – the United States could soon implement some of the bold investments and policies necessary to set the world’s second-largest carbon polluter on a track consistent with the Paris targets.
As Texas Tech climate scientist Katharine Hayhoe put it,
Again and again, assessment after assessment, the IPCC has already made it clear. Climate change puts at risk every aspect of human life as we know it … We are already starting to experience those risks today; but we know what we need to do to avoid the worst future impacts. The difference between a fossil fuel versus a clean energy future is nothing less than the future of civilization as we know it.
Back to the Avengers: They had only one chance in 14 million to save the day, and they succeeded. Time is running short, but policymakers’ odds of meeting the Paris targets remain much better than that. There are no physical constraints playing the role of Thanos in our story; only political barriers stand between humanity and a prosperous clean energy future, although those can sometimes be the most difficult types of barriers to overcome.
Also see: Key takeaways from the new IPCC report