The latest report from the Intergovernmental Panel on Climate Change (IPCC) lays out stark evidence that our planet is in crisis. The impacts are present and apparent: unprecedented heat waves in the Pacific northwest of the United States; deadly flooding in northern Europe and China; and wildfires in northern California, Greece, and Turkey. The troubling signs we see today are overshadowed only by the likelihood of even greater impacts in our future, and the future of our children and grandchildren, if we fail to act.

This report is flush with warnings—and data supporting them—about the world we will live in and the challenges we will face to protect vulnerable communities, help financially strapped nations adapt, and sustain the economies we depend on. Yet one of the main questions raised by this mammoth 3,949-page report is “what should policymakers do about it?”

The first steps are to understand the significance of the IPCC’s efforts, its findings, and the extent to which decisive policy action can meaningfully address the climate crisis.

The IPCC is the United Nations expert body of scientists from around the world, formed in 1988 under the auspices of the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). Yesterday’s IPCC release of the Sixth Assessment Report of IPCC’s Working Group I (WGI) focuses on the physical science of climate change.

While the WGI report and its covers the full state of knowledge on the pace, extent, and drivers of climate change, several conclusions stand out.

Key takeaways from the report

• The contribution of human influence to warming is unequivocal, and the continued pace of warming has led to increased confidence among scientists about the response of the climate system to increasing concentrations of greenhouse gases. While there has long been an overwhelming scientific consensus on the human causes of climate change, this IPCC report is notable for stating the connection in such unequivocal terms. Accumulated evidence allows for greater certainty about the likely pace of future warming: the report also finds a narrower range for estimates of climate sensitivity, which measures the increase in global average temperatures that would result over the long run from a doubling of carbon dioxide concentration in the atmosphere. It’s important to understand that the range of climate sensitivity is not symmetric. Lower-end estimates of climate sensitivity are necessarily bounded by the impacts we are already observing: the fact that the planet has already warmed more than one degree Celsius above pre-industrial levels, well before atmospheric carbon dioxide concentrations have doubled, rules out low values for the climate sensitivity. On the other hand, climate models cannot rule out very high-end estimates of climate sensitivity.
• Second, with continued increases in greenhouse gas emissions, average global temperatures are likely to exceed 1.5 degrees Celsius above pre-industrial levels in the next two decades and projected to reach 2 degrees Celsius by mid-century. Scenarios in which emissions continue to increase after 2030 forecast warming of well above 3 degrees Celsius by the end of the century. These alarming findings about how fast the planet is heating up underscore the urgency of making deep cuts in climate pollution in this decade and achieving net-zero greenhouse gas emissions by the middle of the century at the latest, in order to keep open the possibility of limiting warming to well below 2 degrees, and ideally 1.5 degrees, as called for in the Paris Agreement. The findings also drive home the critical need to protect people from the harmful impacts of climate change by building more resilient communities, governments, and businesses.
• Third, the report concludes that human-influenced changes in weather and climate extremes, including extreme heat, heavy precipitation events, and drought, are already affecting every inhabited region on the planet. Moreover, advances in scientific understanding over the eight years since WGI released its Fifth Assessment Report now allow scientists to clearly attribute the severity and intensity of many individual extreme weather events to climate change with a high degree of confidence. Years ago, it was common to say “no single event provides evidence of climate change.” That is no longer true. The IPCC report concludes that we have enough data and evidence to make strong statements about the degree to which rising concentrations of carbon dioxide, methane, and other greenhouse gasses from human activity have increased the probability of various extreme weather events, raising the likelihood that tropical storms bring more intense rainfall (like that observed in Hurricane Harvey in 2017 and Tropical Storm Imelda in 2019), that heat waves are hotter (including the record-breaking heat wave in Europe in 2019 as well as this year’s heat waves in the Pacific Northwest and Siberia), and that droughts are more severe.
• Looking forward, the report highlights the projected increase in extreme weather events in a range of future emission scenarios:
  • Extreme weather events that used to occur only once or twice per decade or century will be much more frequent in scenarios with continued increases in emissions and therefore warming. For example, heatwaves that formerly occurred once every 50 years will be 40 times as frequent (i.e., likely to occur 40 times in 50 years) in a 4-degree world; extreme heavy precipitation events will be nearly 3 times as frequent as they would be in a climate without human influence, and drought 4 times as frequent. While such events are also projected to be more common under 1.5 and 2 degrees of warming, the increases are much less dramatic.
  • Under intermediate and high emissions scenarios, mean sea level is projected to rise by at least a foot and a half to more than three feet by the end of the century, and a rise of 6 feet by 2100 cannot be ruled out.
  • One of the most arresting sections of the report is Figure SPM.5, which presents a series of maps depicting the changes in extreme weather projected under different levels of warming. A planet that is four degrees above pre-industrial levels—a temperature rise well within the forecast range by the end of the century, if emissions continue to rise—is a planet gone haywire: an Arctic region that is more than 7 degrees Celsius (13 degrees Fahrenheit) hotter; some regions of the world seeing precipitation decline up to 40 percent while others getting wetter by the same proportion; changes in soil moisture levels varying wildly by region.

• The report confirms that the projected rise in temperature is nearly linearly related to carbon dioxide emissions, and that larger increases in temperature lead to more dramatic changes in extreme weather events. As the 2018 IPCC Special Report on Global Warming of 1.5 Degrees found, and as G20 climate ministers recently noted, every fraction of a degree of warming avoided brings enormous benefits in terms of human health and well-being, food production, economic productivity, and ecosystems.

This final point is also critical because it adds emphasis to the point that delayed action leads to real world consequences and greater probability for damage to human health and economic stability. Targets are truly important for planning and agreement, but primarily as a precursor to action.

The way forward for policy

This is usually the point in a blog where I’d write “The good news is…”  But to be frank, this is a report that is very short on good news, precisely because it is an unflinching look at what we are doing to our planet.

If there is a silver lining, it’s that we know the path we need to take to avoid the catastrophic consequences that are so starkly laid out in the report.

Policymakers must collaborate closely with the private sector to develop and deploy every available policy and technological solution: carbon pricing and other policies to cut climate pollution across the economy, investments in a range of technologies including carbon capture and carbon dioxide removal, resources to help communities adapt to a changing climate.

As the world comes together at COP26 this November, all countries have a responsibility to act. Those G20 countries that have yet to do so must come forward with more ambitious 2030 targets and commitments to achieve net zero greenhouse gas emissions. And rich countries must make good on their promise to provide climate finance for those in need.

As I noted in my initial reaction to the Working Group I findings, limiting warming to 1.5 degrees or even 2 degrees Celsius will not be easy, and meeting the challenge will require an all-in effort of global leadership, as well as ambition at the federal, state, and local levels. The imperative for leadership has never been greater.