Urban resilience and the IPCC’s Sixth Assessment Report: WGI the physical science basis

September 2021

The recently released Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment (AR6) Working Group I report provides an assessment of latest evidence on the physical science of climate change. The 4,000-page report was written by 234 contributing authors from 65 countries. Over 14,000 scientific papers were assessed as part of the review process.


The report is significant because it includes research based on the latest Coupled Model Intercomparison Projects (CMIP) projections (CMIP6), and a new set of forward-looking scenarios “Shared Socioeconomic Pathways” (SSPs).[1] The five emissions scenarios range from global net negative and net zero (SSP1-1.9 and SSP1-2.6), to emissions doubling by 2050 and 2100 (SSP5-8.5 and SSP3-7.0 respectively), compared to current levels. It presents the most up-to-date physical understanding of the climate system and climate change.[2]


The report confirms a lot of what we already knew: that the earth is warming, that human-produced greenhouse gas emissions are responsible for global temperature rise, and that this is driving extreme weather events in every region of the world. The report is clear that climate change is not an issue for the future. Far from it, we are already living with the dangerous impacts of climate change.[3],[4] 


The authors also make clear that warming is set to continue under all scenarios until mid-century and that we are on track to reach 1.5˚C of warming by 2040. This sends a clear signal to invest in urban climate resilience as extreme weather events and chronic climate impacts will continue to worsen until global emissions reach net zero.


What does the report mean for Asia’s cities?


The AR6 WGI report includes more regional information than previous IPPC report releases, with the latest climate models providing a clearer picture of projected impacts across the globe. The IPCC has this year released a set of regional fact sheets summarising some of the projections in the report. It has also created an online Interactive Atlas presenting the report’s findings at the global and regional levels.


In Asia, heat and flooding remain the predominant risks. The report is able to link extreme weather events – such as heatwaves and extreme rainfall – to human-induced climate change more directly, thanks to advances in ‘attribution science’. According to the report, anthropogenic climate change is likely responsible for more than half of the increase in heavy rainfall events over many areas since the 1950s.


Heat extremes have increased over the past 100 years a trend that is expected to continue over the coming decades (high confidence). This is especially significant for cities as continued urbanization and development have amplified the warming effects within urban areas (very high confidence).


As Figure 1 shows the difference in observed warming trends between cities and their surroundings can partly be attributed to urbanization (very high confidence). Here we see that urbanization is responsible for almost 50 % of warming in Thailand’s cities, over 25% of warming in Ho Chi Minh, Viet Nam, and over 80% of the temperature rise in Kolkata, India.

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Figure 1: Trend in global (near) surface air temperature (CRUTS, 1950-2018) IPCC 2021, Figure 10.3.


The amplifying effect of heat in cities also means that extreme temperatures are more extreme than in surrounding areas, this is particularly apparent for night-time extreme temperatures (high confidence). Future urbanization will amplify the projected air temperature change in cities regardless of the characteristics of the background climate, resulting in a warming signal on minimum temperatures that could be as large as the global warming signal (very high confidence).


Figure 2: Why cities are hotspots of global warming. Contributing factors to climate risk in cities. IPCC 2021: Urban heat island effect: Box 10.3, FAQ 10.2.

Figure 2 shows why cities amplify heat extremes. Here the IPCC indicates that adding ‘blue’ and ‘green’ infrastructure such as lakes, parks, and trees can help to reduce urban temperatures. Looking ahead the IPCC expects more frequent occurrence of extreme climate events, such as heatwaves, with more hot days and warm nights adding to heat stress in cities (very high confidence). The report also indicates that rising temperatures will also exacerbate air pollution problems in cities (medium confidence).


Average and heavy precipitation levels will also increase over much of Asia (high to medium confidence), which will likely lead to more river and surface flooding. Relative sea level around Asia has also increased faster than global average and the science suggests that regional-mean sea level will continue to rise (high confidence), putting more cities at risk of coastal inundation.


Sea levels are projected to rise in most coastal settlements (high confidence). The combination of extreme sea level, increased by both sea level rise and storm surge, and extreme rainfall and river flow events will increase the probability of flooding (high confidence). The report also indicates that an increase in surface water flooding from extreme rainfall events is expected in urban areas (high confidence).


Finally, the report also notes that irreversible changes to Asia’s water stores are also expected. Glaciers are declining and permafrost is thawing. Seasonal snow duration, glacial mass, and permafrost area will decline further by the mid-21st century (high confidence). Glacier runoff in the Asian high mountains will increase up to mid-21st century (medium confidence), and subsequently runoff may decrease due to the loss of glacier storage putting pressure on water supplies for many cities who depend on them.

The recently released Physical Science Basis report will be followed by two further reports to be released in the first half of 2022. The next report, from Working Group III, covers Mitigation of Climate Change, the third from Working Group II covers Impacts, Adaptation and Vulnerability. A fourth overall Synthesis Report will also be published in 2022.  


[1] Replacing the previous set of Representative Concentration Pathways (RCPs).

[2] IPCC (2001) AR6 Working Group I report: The Physical Science Basis:

[3] IPCC, 2021 (B.3) “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.”

[4] IPCC, 2021 (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.