The IPCC Report: Context

What is the IPCC?

The Intergovernmental Panel on Climate Change (IPCC) is the UN Agency for assessing the science related to climate change. It was founded in 1988 and is an intergovernmental organization with 195 member countries.

What is this report?

IPCC prepares comprehensive Assessment Reports about knowledge on climate change, its causes, potential impacts, and response options. It also produces Special Reports, which are an assessment on a specific issue.

This report is the sixth IPCC Assessment Report, which means it is essentially a summary of the current state of climate science. IPCC released its first Assessment Report in 1990. The report itself is 3,949 pages long and cites 14,000 studies. But you can also read the 42-page policymakers’ summary, which is what I did.

The report draws on five different emissions scenarios to arrive at its findings: very low, low, intermediate, high, and very high.

What makes this report different?

1. It Is the most definitive report so far

It’s the most definitive of any IPCC report. The report declares that:

“It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.”

This is actually a huge deal. 195 countries must agree to the findings, which makes the IPCC reports more conservative than is prudent.

Unequivocal has a precise definition in the IPCC, because the IPCC uses calibrated language with statistical meanings. “Unequivocal” means that there is a “100 percent probability”—there is no room to question it. The term “unequivocal” appears in the IPCC report 32 times.

Why was the IPCC so confident in sharing unequivocal conclusions in this report? We have more, and better, evidence about climate change. There are more and better observations, models, and statistical analysis of climate change.

2. It shows that climate change is happening now

This is the first IPCC report to establish that extreme weather events are happening because of climate change.

The report also finds that sea level rise is worse than they previously thought. Under an intermediate emissions scenario, sea levels are expected to rise two feet by 2100—and if a catastrophic glacier collapse occurs, the rise could be as much as 6.5 feet.

3. It concludes that many climate effects are irreversible and will continue to get worse even after we reach net zero

Some climate effects are probably reversible, such as ocean acidification and the rise of land temperatures.

But some aren’t, and that includes sea level rise. Once it occurs, it will be irreversible for at least centuries, maybe millennia. And it may continue even after emissions reductions for decades or centuries. The Arctic will also continue warming at more than twice the rate of the rest of the world (as it is now) for the rest of the century.

4. It emphasizes that we have very little time to act if we want to stay below 1.5 degrees of warming

We have already experienced 1.1 degrees of warming (and, actually, it’s 2 degrees in Canada because we’re farther north).

The world is likely to exceed 1.5 degrees of warming by 2040 even if humanity cuts emissions as quickly as is plausible: there is already enough GHG in the atmosphere to raise the planet’s temperature by 1.5 degrees the cooling effect of air pollution is keeping temperatures down.

Current policies suggest that we are headed for 3 degrees of warming by 2100, but we could conceivably get back under 1.5 degrees if we act decisively.

Why does this report matter?

Because the report is so conclusive, it could be used as evidence in court cases seeking to force governments and companies to take greater climate action. IPCC reports have been used by courts in the past. But this report draws on attribution science linking extreme weather to emissions, and that could be a game-changer.

There is a big climate meeting (UN Conference of the Parties or COP 26) happening in Scotland this November. The IPCC report could play a major role in that meeting. Countries are already being asked to submit bolder climate plans before the meeting.

Main Findings of the IPCC Report

A. The Current State of the Climate

A.1: It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere, and biosphere have occurred.

· Global surface temperatures have already risen an estimated 1.07 degrees.

· Global sea levels have increased by an estimated 0.2m between 1901 and 2018

A.2.: The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to thousands of years.

· CO2 levels are higher than at any time in at least 2 million years

A.3: Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.

· Hot extremes have become more frequent and more intense across most land regions since the 1950s

· The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area (for which sufficient data are available)

· The global proportion of major (Category 3-5) tropical cyclone occurrence has increased in the last four decades and the tropical cyclones are coming higher northward.

· The frequency of extreme weather events like heatwaves, droughts, and fire weather has increased since the 1950s

B. Possible Climate Futures

B.1: Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios. Global warming of 1.5 and 2 degrees will be exceeded in the 21st century unless deep reductions in CO2 and other GHG emissions occur in the coming decades

· Under the low emissions scenario, 1.0-1.8 degrees of warming is expected by 2100. Even under the low emissions scenario, we are likely to exceed 1.5 degrees of warming.

· Under the intermediate emissions scenario, 2.1-3.5 degrees of warming is expected

· Under the very high scenario, 3.3-5.7 degrees of warming is expected

· The last time global surface temperature was sustained at or above 2.5 degrees higher than 1850-1900 was over 3 million years ago

B.2.: Many changes in the climate system become larger in direct relation to increasing global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover, and permafrost.

· Every additional 0.5 degrees of global warming causes increases in the intensity and frequency of hot extremes, heavy precipitation, and drought.

· Extreme daily precipitation events are projected to intensify by about 7% for each 1 degree of global warming

· The Arctic is likely to be practically sea ice free in September at least once before 2050

B.3: Continued global warming is projected to further intensity the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events.

· Depending on how much we emit, average annual global land precipitation is expected to increase by 0-13% (0-5% for very low emissions, 1.5-8% for intermediate, 1-13% for very high)

B.4: Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere

B.5: Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets, and global sea level.

· Glaciers will continue to melt for decades or centuries

· Sea levels will continue to rise over the 21st century, and maybe for centuries

C. Climate Information for Risk Assessment and Regional Adaptation

C.1: Natural drivers and internal variability will modulate human-caused changes, especially at regional scales and in the near term, with little effect on centennial global warming. These modulations are important to consider in planning for the full range of possible changes

C.2: With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers. Changes in several climatic impact-drivers would be more widespread at 2-degree compared to 1.5-degree global warming and even more widespread and/or pronounced for higher warming levels

C.3: Low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, some compound extreme events and warming substantially larger than the assessed very likely range of future warming cannot be ruled out and are part of a risk assessment.

D. Limiting Future Climate Change

D.1: From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.

· Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.45 global surface temperature increase

· From 1850-2019, 2390 GtCO2 of anthropogenic CO2 was emitted

· If global net negative CO2 emissions were to be achieved and be sustained, the global surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia (e.g., sea level)

D.2: Scenarios with very low or low GHG emissions lead within years to discernible effects on GHG and aerosol concentrations, and air quality, relative to high and very high GHG emissions scenarios. Under these contrasting scenarios, discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climatic impact-drivers