Scientists from around the world have once again raised the spectre of a bleak future for humanity if emissions of carbon dioxide and other greenhouse gases aren't brought under control.
In its sixth assessment report, the Intergovernmental Panel on Climate Change has declared that the "scale of recent changes across the climate system as a whole...are unprecedented over many centuries to many thousands of years".
"Human-induced climate change is already affecting many weather and climate extremes in every region across the globe," the report states. "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 the Fifth Assessment Report (AR5)."
Under all emissions scenarios that were considered, the targets in the 2015 Paris Agreement will be exceeded this century without "deep reductions" in the release of greenhouse gases. The legally binding treaty set a target of a 1.5°C average increase from the start of the Industrial Revolution, with the overall goal to limit global warming to well below 2°C.
According to the IPCC, "paleoclimate evidence and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3°C, with a narrower range compared to AR5."
Multiple changes across planet
The report was released in a summer when unprecedented and deadly heat domes have occurred in several parts of the Northern Hemisphere and wildfires are raging in British Columbia, the western United States, Greece, and other parts of the world.
"Many changes in the climate system become larger in direct relation to increasing global warming," the IPCC states. "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.
"Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events."
Moreover, the report points out that as carbon dioxide emissions increase, carbon sinks, including large forests and oceans, are anticipated to be "less effective" in slowing the accumulation of carbon dioxide in the atmosphere.
"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," it declares.
There are dozens of drafting authors and contributors to the report, including seven listed as being from Canada: Gregory M. Flato, John C. Fyfe, Nathan P. Gillett, Xuebin Zhang, Kirsten Zickfield (also from Germany), Alejandro Di Luca (also from Australia and Argentina), and Shubba Sathyendranath (also listed as being from the United Kingdom and an overseas citizen of India).
The report drives home the point that climate change is intensifying and it's having large effects already. Heat waves will increase in intensity and occur more frequently in the future.
"The likely range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C," the summary for policymakers states. "It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by –0.1°C to 0.1°C, and internal variability changed it by -0.2°C to 0.2°C."
The jet streams travel through the troposphere and the slowing of these air currents have been linked to the recent heat domes in the Northern Hemisphere.
"It is very likely that well-mixed GHGs were the main driver of tropospheric warming since 1979, and extremely likely that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s," the summary for policymakers states.
Global averaged precipitation over land has likely increased since 1950, it adds, and with "medium confidence", the scientists state that it has risen at a faster rate since the 1980s.
"It is likely that human influence contributed to the pattern of observed precipitation changes since the mid-20th century, and extremely likely that human influence contributed to the pattern of observed changes in near-surface ocean salinity."
In addition, human influence is "very likely the main driver of the global retreat of glaciers since the 1990s and the decrease in Arctic sea ice area between 1979-1988 and 2010-2019 (about 40% in September and about 10% in March".
Moreover, human influence also "very likely contributed to the decrease in Northern Hemisphere spring snow cover since 1950" and "the observed surface melting of the Greenland Ice Sheet over the past two decades".
However, there has been "no significant trend in Antarctic sea ice area from 1979 to 2020 due to regionally opposing trends and large internal variability".
"It is virtually certain that the global upper ocean (0–700 m) has warmed since the 1970s and extremely likely that human influence is the main driver," it adds. "It is virtually certain that human-caused CO2 emissions are the main driver of current global acidification of the surface open ocean. There is high confidence that oxygen levels have dropped in many upper ocean regions since the mid-20th century, and medium confidence that human influence contributed to this drop."
Sea levels are expected to continue to rise, possibly by up to two metres by the end of the 21st century and by up to five metres by 2150, though these are the upper thresholds. Other scenarios peg sea-level rise at between 0.5 and one metre by 2100.
For more detailed reports, visit the IPPC website. Here are some of the responses floating over social media: