IPCC report issues dire warning on climate change

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New Delhi, 11 Aug (Indrajit Bose) – Global warming of 1.5 degrees C and 2 degrees C will be exceeded during the 21st century unless deep reductions in carbon dioxide (CO2) and other greenhouse gas (GHG) emissions occur in the coming decades, states the latest report by the Intergovernmental Panel on Climate Change (IPCC).

Member governments of the IPCC adopted the Summary for Policymakers (SPM) of the “Climate Change 2021: The Physical Science Basis” and approved the underlying assessment report on Friday, 6 August at its 54th Session, held virtually for two weeks.

The report is IPCC Working Group 1’s (WG 1) contribution to the 6th Assessment Report, expected to be out in 2022.

The IPCC convened the approval session of the SPM virtually for the first time due to the ongoing COVID-19 pandemic.

The SPM consists of four sections: (i) The Current State of the Climate; (ii) Possible Climate Futures; (iii) Climate Information for Risk Assessment and Regional Adaptation; and (iv) Limiting Future Climate Change.

The report conveyed a number of key messages in the form of headline statements under each of the four sections.

(Headline statements comprise conclusions of the technical and scientific assessment undertaken and are primarily aimed for media use and the public, but are regarded by many governments as sensitive.)

The SPM highlights 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.”

The SPM also highlights that “each of the last four decades has been successively warmer than any decade that preceded it since 1850. Global surface temperature in the first two decades of the 21st century (2001-2020) was 0.99 degrees C higher than 1850-1900. Global surface temperature was 1.09 degrees C higher in 2011-2020 than 1850-1900, with larger increases over land than over the ocean”.

The SPM states that “global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years. Temperatures during the most recent decade (2011-2020) exceed those of the most recent multi-century warm period, around 6500 years ago” and that the “Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3000 years”.

The report also states that, “Past GHG emissions since 1750 have committed the global ocean to future warming”.

The SPM states that “hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s”; “cold extremes (including cold waves) have become less frequent and less severe”; “the frequency and intensity of heavy precipitation events have increased, as have droughts.”

The report reveals further that the global proportion of major (Category 3-5) tropical cyclone occurrence has likely increased over the last four decades and “event attribution studies and physical understanding indicate that human- induced climate change increases heavy precipitation associated with tropical cyclones but data limitations inhibit clear detection of past trends on the global scale”.

With respect to possible climate futures, the report assesses the climate response to five illustrative scenarios.

“They start in 2015, and include scenarios with high and very high GHG emissions and CO2 emissions that roughly double from current levels by 2100 and 2050, respectively, scenarios with intermediate GHG emissions and CO2 emissions remaining around current levels until the middle of the century, and scenarios with very low and low GHG emissions and CO2 emissions declining to net zero around or after 2050, followed by varying levels of net negative CO2 emissions,” the SPM explains.

“Compared to 1850-1900, global surface temperature averaged over 2081-2100 is very likely to be higher by 1.0 degrees C to 1.8 degrees C under the very low GHG emissions scenario considered, by 2.1 degrees C to 3.5 degrees C in the intermediate scenario and by 3.3 degrees C to 5.7 degrees C under the very high GHG emissions scenario,” states the SPM.

With respect to global warming of 2 degrees C, the SPM states that it would “extremely likely be exceeded in the intermediate scenario”.

“Under the very low and low GHG emissions scenarios, global warming of 2 degrees C is extremely unlikely to be exceeded, or unlikely to be exceeded. Crossing the 2 degrees C global warming level in the mid-term period (2041-2060) is very likely to occur under the very high GHG emissions scenario, likely to occur under the high GHG emissions scenario, and more likely than not to occur in the intermediate GHG emissions scenario,” the SPM reads.

The SPM also states that “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, proportion of intense tropical cyclones as well as reductions in Arctic sea ice, snow cover and permafrost”.

The SPM adds that:

* the land surface will continue to warm more than the ocean surface;

* with every additional increment of global warming, changes in extremes continue to become larger;

* there will be an increasing occurrence of some extreme events … with additional global warming, even at 1.5 degrees C of global warming;

* the Arctic is projected to experience the highest increase in the temperature of the coldest days, at about 3 times the rate of global warming and is likely to be “practically sea ice free” in September;

* heavy precipitation events will intensify and become more frequent in most regions with additional global warming;

* intense tropical cyclones (categories 4-5) and peak wind speeds of the most intense tropical cyclones are projected to increase at the global scale with increasing global warming;

* the global water cycle will continue to intensify as global temperatures rise, with precipitation and surface water flows projected to become more variable over most land regions within seasons and from year to year;

* a warmer climate will intensify very wet and very dry weather and climate events and seasons, with implications for flooding or drought; and

* monsoon precipitation is projected to increase in the mid- to long-term at global scale, particularly over South and Southeast Asia, East Asia and West Africa apart from the far west Sahel.

“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,” states the SPM.

In relation to sea level rise, the SPM states that, “Over the next 2000 years, global mean sea level will rise by about 2-3 m if warming is limited to 1.5 degrees C; 2-6 m if limited to 2 degrees C; and 19-22 m with 5 degrees C of warming, and it will continue to rise over subsequent millennia.”

With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers (CIDs).

(Examples of CIDs include Heat and Cold; Wet and Dry; Wind; Snow and Ice; Air pollution; Coastal drivers such as relative sea level, coastal flood, coastal erosion, marine heatwave and ocean acidity).

Changes in several climatic impact-drivers would be more widespread at 2 degrees C compared to 1.5 degrees C global warming and even more widespread and/or pronounced for higher warming levels, the SPM states.

Many regions are projected to experience an increase in the probability of compound events with higher global warming.

In particular, concurrent heatwaves and droughts are likely to become more frequent.

Concurrent extremes at multiple locations become more frequent, including in crop-producing areas, at 2 degrees C and above compared to 1.5 degrees C global warming, states the SPM.

Under the section on limiting future emissions, the SPM states that, “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 (methane) emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.”

The report reaffirms that there is a “near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause”.

“Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27 degrees C to 0.63 degrees C increase in global surface temperature with a best estimate of 0.45 degrees C. This quantity is referred to as the transient climate response to cumulative CO2 emissions (TCRE).”

“This relationship implies that reaching net zero anthropogenic CO2 emissions is a requirement to stabilize human- induced global temperature increase at any level, but that limiting global temperature increase to a specific level would imply limiting cumulative CO2 emissions to within a carbon budget”.

(Net zero is explained as a “condition in which anthropogenic carbon dioxide (CO2) emissions are balanced by anthropogenic CO2 removals over a specified period” and refers to emissions at the global level).

(Carbon budget in the SPM is explained as: “the maximum amount of cumulative net global anthropogenic CO2 emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date. Historical cumulative CO2 emissions determine to a large degree warming to date, while future emissions cause future additional warming. The remaining carbon budget indicates how much CO2 could still be emitted while keeping warming below a specific temperature level.”)

The SPM also states that, “Over the period 1850-2019, a total of 2390 GtCO2 of anthropogenic CO2 was emitted. Remaining carbon budgets have been estimated for several global temperature limits and various levels of probability, based on the estimated value of TCRE and its uncertainty, estimates of historical warming, variations in projected warming from non-CO2 emissions, climate system feedbacks such as emissions from thawing permafrost, and the global surface temperature change after global anthropogenic CO2 emissions reach net zero”.

The report adds that, “Anthropogenic CO2 removal (CDR) leading to global net negative emissions would lower the atmospheric CO2 concentration and reverse surface ocean acidification” and, “If global net negative CO2 emissions were to be achieved and be sustained, the global CO2-induced surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia. For instance, it would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions.”

Some of the other key messages from the SPM include:

* 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 many thousands of years.

* 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 (5th Assessment Report).

* Improved knowledge of climate processes, paleoclimate evidence and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3 degrees C with a narrower range compared to AR5.

* 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.

* 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.

* 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.

* 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.

* 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 risk assessment.

* 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.

* Scenarios with low or very low GHG emissions lead within years to discernible effects on greenhouse gas 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.

Third World Network