New climate studies: Worse risks at 2°C rise, higher rise likely

Desdemona Despair

By Megan Gannon
21 April 2016

(Eos) – To mark Earth Day, leaders from more than 165 countries are expected to gather at the United Nations tomorrow for a ceremony to sign the Paris climate agreement, reached last December. Under the historic deal, each country has set targets to reduce greenhouse gas emissions, with the ultimate goal to keep the rise in global temperatures below 2°C above the preindustrial average.

However, new research  suggests that the emissions caps might not be low enough to prevent damaging impacts for parts of the world that are vulnerable to climate change. A study presented today at the 2016 European Geosciences Union (EGU) meeting in Vienna, Austria, shows big differences between a 2°C warmer world and a 1.5°C warmer world.

The study, also published today in Earth System Dynamics [pdf], shows that tropical regions would be especially hard hit by a 2°C temperature increase. Warm spells would last up to 50% longer, resulting in corn and wheat yields half what they would be under a 1.5°C increase, the researchers found. What’s more, a 2°C temperature increase by 2100 would also mean that all coral reef ecosystems in tropical regions would be at risk of degradation due to coral bleaching, according to the study led by Carl Schleussner of Climate Analytics in Germany. Greater Reductions Needed

To limit temperature increases to just 1.5°C, countries may need to strengthen their emission reduction pledges significantly. Even if the current Paris commitments are met and extended beyond 2030, global temperatures are on track to rise 3°C above the preindustrial average, said Massachusetts Institute of Technology climate scientist Erwan Monier.

He collaborated in another study, also presented at the EGU meeting this week, that combined a human activity model with a climate model to look at five different global warming scenarios through 2100. His team found that there is only a 5% probability that the Paris agreement will keep global temperatures below 2°C, even with the most optimistic outlook.

Nonetheless, Monier told Eos that it is still possible to limit temperatures to 2°C by the end of the century. However, that would require major changes in policy. “We’re not on that path right now, but it’s totally achievable,” he said. “I think most people know some policy tools that would get us there, like a carbon tax. But there’s unwillingness to actually use those.” Possible 2.7°C Rise Scrutinized

Other researchers have focused on tipping points for severe climate impacts that may lie beyond 2°C. Climate scientist Robert DeConto of the University of Massachusetts Amherst published a paper in Nature last month that found that the Antarctic ice sheet would barely contribute to sea level rise if the average global temperature rise stayed below 2°C. Since writing that paper, DeConto has applied his same model to a temperature increase of 2.7°C. At a press conference at the EGU meeting today, DeConto explained that his preliminary results suggest that the Antarctic ice sheet would contribute about 80 centimeters of potential sea level rise in a 2.7°C warmer world. DeConto chose to scrutinize the effects of a 2.7°C increase because the Climate Action Tracker had warned ahead of the Paris agreement that the world is headed for that level of warming by 2100 even if governments fully implement their climate action pledges. [more]

New Climate Studies: Worse Risks at 2°C Rise, Higher Rise Likely

ABSTRACT: Robust appraisals of climate impacts at different levels of global-mean temperature increase are vital to guide assessments of dangerous anthropogenic interference with the climate system. The 2015 Paris Agreement includes a two-headed temperature goal: “holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 °C”. Despite the prominence of these two temperature limits, a comprehensive overview of the differences in climate impacts at these levels is still missing. Here we provide an assessment of key impacts of climate change at warming levels of 1.5 °C and 2 °C, including extreme weather events, water availability, agricultural yields, sea-level rise and risk of coral reef loss. Our results reveal substantial differences in impacts between a 1.5 °C and 2 °C warming that are highly relevant for the assessment of dangerous anthropogenic interference with the climate system. For heat-related extremes, the additional 0.5 °C increase in global-mean temperature marks the difference between events at the upper limit of present-day natural variability and a new climate regime, particularly in tropical regions. Similarly, this warming difference is likely to be decisive for the future of tropical coral reefs. In a scenario with an end-of-century warming of 2 °C, virtually all tropical coral reefs are projected to be at risk of severe degradation due to temperature-induced bleaching from 2050 onwards. This fraction is reduced to about 90 % in 2050 and projected to decline to 70 % by 2100 for a 1.5 °C scenario. Analyses of precipitation-related impacts reveal distinct regional differences and hot-spots of change emerge. Regional reduction in median water availability for the Mediterranean is found to nearly double from 9 % to 17 % between 1.5 °C and 2 °C, and the projected lengthening of regional dry spells increases from 7 to 11 %. Projections for agricultural yields differ between crop types as well as world regions. While some (in particular high-latitude) regions may benefit, tropical regions like West Africa, South-East Asia, as well as Central and northern South America are projected to face substantial local yield reductions, particularly for wheat and maize. Best estimate sea-level rise projections based on two illustrative scenarios indicate a 50 cm rise by 2100 relative to year 2000-levels for a 2 °C scenario, and about 10 cm lower levels for a 1.5 °C scenario. In a 1.5 °C scenario, the rate of sea-level rise in 2100 would be reduced by about 30 % compared to a 2 °C scenario. Our findings highlight the importance of regional differentiation to assess both future climate risks and different vulnerabilities to incremental increases in global-mean temperature. The article provides a consistent and comprehensive assessment of existing projections and a good basis for future work on refining our understanding of the difference between impacts at 1.5 °C and 2 °C warming.

Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 °C and 2 °C