Escalating climate change risks threaten cities globally. Urgent action for adaptation and mitigation is needed to ensure a sustainable future.
In the realm of climate change, the impact on ecosystems and our living environment is profound. Drawing from the IPCC’s sixth assessment report, this talk underscores the urgency of understanding these risks and their implications.
The decade spanning the last ten years witnessed a global surface temperature that surpassed the average from 1850 to 1900 by 1.1°C. This warming trend is unparalleled in the past 2,000 years, with each successive decade proving hotter than any previous one since 1850. Human activity, primarily greenhouse gas emissions, has been identified as the leading factor behind this warming.
Visible consequences of global warming
The signs of change in the climate system are conspicuous, widespread, and escalating. The atmosphere is warming and growing more humid, with significant shifts observed in large-scale climatic circulation patterns and the trajectories of tropical storms. This pattern extends to the cryosphere, where reductions in snow and ice are pervasive. The Arctic’s ice area and thickness are diminishing, permafrost is warming and retreating, glaciers worldwide are retreating, and Greenland and Atlantic ice are experiencing losses due to melting and ice shelf degradation.
Meanwhile, ocean warming is evident, accompanied by frequent marine heatwaves and shifts in multiple oceanic characteristics such as salinity, acidity, and oxygen levels. These transformations lead to shifts in marine species distribution. Sea level rise, a composite effect of land ice loss, thermal expansion, and oceanic changes, is accelerating at a rate of around 3.7 millimeters annually. On land, surface air temperature is rising faster than over oceans, causing shifts in climate zones.
Intensification of extreme events
In many terrestrial areas, availability of water is reduced during dry periods, due to escalated evaporation and plant transpiration. Shifts in species distribution, both upwards in elevation and towards cooler climes, are occurring across land ecosystems. Moreover, human-induced climate change is fostering intensification of extreme weather and climate events on a global scale, manifesting as elevated land temperatures and increased precipitation extremes.
Future warming hinges on emission scenarios and greenhouse gas concentration. The consensus is that over the next two decades, the global surface temperature average over 20 years will likely reach 1.5°C. This indicates a 50% probability of annual global mean surface temperatures exceeding this threshold by 2030. Stricter emission controls are essential to restrict global surface temperature increase to below 2°C by the century’s end.
Bigger risks for cities
However, these goals are unlikely to be met unless substantial reductions in greenhouse gas emissions transpire in coming decades. Continued warming is anticipated to amplify the global water cycle, leading to heightened monsoon precipitation and more severe wet and dry events. Extreme events, especially heatwaves, heavy rainfall, and droughts, will gain frequency and intensity with increased warming.
Cities, central to human habitation, face heightened risks. Urban heat island effects augment warming in urban areas, and cities will experience temperature increases exceeding 1.5°C ahead of non-urban regions. Intensified urbanization amplifies flooding risks from heavier rainfalls, as urban surfaces modify natural drainage systems.
Could cities bring solutions?
Low-lying islands and coasts confront rapid sea-level rise, compounded by storm surges and extreme events. Coastal floods could trigger widespread devastation to infrastructure, housing, transportation, and energy systems. Urbanization’s interaction with climate events will exacerbate heat stress within cities, perpetuating urban heat island effects.
The prognosis of rising temperatures, sea levels, and extreme events accentuates the necessity for comprehensive adaptation strategies and mitigation measures. Furthermore, cities plays a crucial role as both contributors to climate change and potential hubs for solutions through innovation, adaptation, and sustainable development. Through a deeper understanding of the climate system, cities can pave the way toward a resilient and sustainable future.
Panmao Zhai, Chinese Academy of Meteorological Sciences