Half a degree Celsius can significantly impact how various regions experience climate change. This conclusion stems from a groundbreaking study published in Earth System Dynamics, the first to compare the consequences of a 1.5°C world versus a 2°C world.
Both temperature thresholds are recognized in the Paris Agreement as potential upper limits for global warming since pre-industrial times. However, detailed scientific comparisons of these thresholds have been limited. This research highlights the significant differences between a 1.5°C and a 2°C warming scenario. For instance, an extra 0.5°C could lead to a 10cm rise in global sea levels by 2100, double the water shortages in the Mediterranean, and extend tropical heatwaves by a month. The difference is also crucial for coral reefs, with nearly all at high risk of bleaching at 2°C warming.
The Paris Agreement’s “Two-Headed Goal”
The Paris Agreement, adopted in December 2015, established a “two-headed” temperature goal: keeping the global average temperature “well below 2°C” and “pursuing efforts” to limit the increase to 1.5°C.
The inclusion of 1.5°C acknowledges the vulnerability of low-lying island nations and the high risks faced by coral reef and Arctic ecosystems at even lower temperature increases. However, the specific mention of 1.5°C caught the scientific community off guard, as there was a lack of comprehensive research on the impacts of a 1.5°C rise.
In response, the IPCC confirmed a dedicated special report on the 1.5°C goal, published in 2018. The study discussed here, initiated in 2014, predates the Paris conference and directly addresses the UN’s call for research comparing the 1.5°C and 2°C long-term goals.
Comparing a 1.5°C and 2°C World: Impacts Vary by Region
This study utilizes 11 climate models to compare the impacts of 1.5°C and 2°C warming on extreme weather, water availability, crop yields, sea level rise, and coral reefs – both globally and across 26 regions. The findings reveal stark regional variations in warming rates and subsequent impacts.
Heat Extremes and Rainfall
Tropical regions experience the most dramatic differences in heat extremes. Heatwaves could last three months at 2°C warming, compared to two months at 1.5°C. This 0.5°C difference represents a shift from current natural variability to a new climate regime, particularly in the tropics. High northern latitudes are projected to see significant increases in heavy rainfall.
Water Scarcity and Sea Level Rise
Water scarcity in the Mediterranean could be twice as severe at 2°C compared to 1.5°C. Global sea levels are projected to rise 50cm by 2100 with 2°C warming, versus 40cm with 1.5°C.
Coral Reefs at Risk
A 2°C warming puts 98% of coral reefs at risk of bleaching from 2050 onwards, compared to 90% at 1.5°C.
Impacts on Crop Yields
Climate change impacts on crops are complex, varying by crop type and region. While higher CO2 levels might positively affect yields in high-latitude regions, this effect remains uncertain. Tropical regions, including West Africa and Southeast Asia, are likely to experience the most significant declines in wheat and maize yields.
From Research to Action: The Challenge of Limiting Warming
The significant differences between a 1.5°C and 2°C world underscore the urgency of mitigating climate change. While limiting warming to 1.5°C is preferable, achieving this goal presents immense challenges. Current emission levels would deplete the remaining carbon budget for 1.5°C within a few years. Reaching this target likely requires overshooting it and then employing negative emissions technologies to reduce atmospheric CO2.
The feasibility and potential consequences of such overshoot scenarios remain critical research areas. This study provides crucial scientific evidence to inform policy decisions regarding climate change mitigation, highlighting the need for ambitious action to limit warming and minimize its devastating impacts.
