Timely action needed to meet climate targets

The Paris Agreement leaves a lot of leeway for the contributing countries, when to exactly start the emission reductions -- "more than the climate system allows for meeting ambitious climate targets," says Patrik Pfister from the Oeschger Centre for Climate Change Research of the University of Bern. Together with climate physicist Thomas Stocker from the Oeschger Centre, Pfister has carried out a study quantifying the consequences of delaying global emission reductions. The study has now been published in the open access journal "Environmental Research Letters."

Peak warming increases rapidly while emission reductions are delayed

The scientists employ their climate model to estimate the peak warming caused by ongoing CO2 emissions. As long as emissions continue to increase, this future peak warming increases much faster than observed warming, namely 3 to 7.5 times as fast. "Short-term variations in the current warming rate could distract us from the urgency of the problem," says Patrik Pfister, lead author of the study. Due to the inertia of the Climate System and the long atmospheric lifetime of CO2, "delaying emission reductions by 10 years causes an additional increase in peak warming of 0.3 to 0.7°C," Pfister continues. In 10 years without global reductions, a 2.5°C target will have become about as ambitious as the 2°C target is today. Little time remains to initiate reductions, if the Paris Agreement is to be met.

Existential importance for islands and coastal cities

Following the atmospheric warming, the ocean also warms globally and expands in the process. This thermal expansion is a major contributor to sea level rise, and increases drastically while emission reductions are delayed. "Until emission reductions start, the long-term thermal expansion increases even 7 to 25 times as fast as the now observed thermal expansion," Pfister cites from the study. A decade of delay in global emission reductions increases the long-term sea level rise by a total of roughly 0.4 to 1.2 meters, depending on the achievable rate of emission reductions. "For islands and coastal cities, the timing and rate of global emission reductions is therefore of existential importance," says Pfister.

Furthermore, ongoing emissions also cause ocean acidification, with substantial impacts on marine ecosystems. For example, the acidification diminishes the extent of of ocean areas that provide ideal chemical conditions for the growth of tropical coral reefs. A near-complete loss of such areas becomes imminent if emission reductions are delayed by few years to decades, again depending on the achievable reduction rate.

"The results of our study underscore the urgency of action," says Thomas Stocker, co-author of the study and past Co-Chair of the Intergovernmental Panel on Climate Change. "With every decade of delaying global emission reductions, we lose roughly 0.5°C of climate target," Stocker remarks. This means that the most ambitious targets already become unachievable within the next few years.