Tipping Risks Call for Stringent Climate Policy

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February 9, 2017

Climate scientists have worked for years to develop predictive models, but some elements are still very uncertain for prediction. In our work we analyzed five major risky climate factors that could accelerate climate change and lead to irreversible and/or abrupt changes to the natural system. These factors are:

  1. Meltdown of the Greenland ice sheet
  2. Collapse of the West Antarctic ice sheet
  3. Collapse of Atlantic Meridional Overturning Circulation (AMOC)
  4. Dieback of the Amazon forest, and
  5. Shift to a more persistent El Nino regime.

These potential events are called "tipping risks", and they are often correlated: one tipping event's occurrence may increase or decrease other tipping events' probability of occurrence. For example, if the Greenland ice sheet melts, then it will increase the probability of collapse of AMOC, due to increasing freshwater influx from melting ice sheet to the North Atlantic. Similarly, collapse of AMOC will in turn warm up the Southern Ocean and then increase the probability of collapse of the West Antarctic ice sheet.

Once these tipping events happen, they are often irreversible and could generate significant economic damages. For example, if the Greenland or Antarctic ice sheets melt, it may take millennia to regenerate them. The resulting sea level rise could be catastrophic, as a complete meltdown of Greenland ice sheet will lead to about 7 meters of sea level rise, and a complete meltdown of West Antarctic ice sheet will lead to about 3.3 meters of sea level rise. Another example: if dieback of the Amazon rainforest happens, we are not only losing this major carbon sink, but also endangering the planet's richest ecosystem which is home to one in ten known species in the world, including several million insect species, tens of thousands of plants, and thousands of birds and mammals.

We have quantified the interdependencies among these tipping risks using a state-of-the-art integrated assessment model of economy and the climate. Our conclusion is that a policymaker wishing to account for the full costs from these combined risks and social impacts would recommend a climate policy with immediate and massive carbon emission reductions, aiming for a complete stop of carbon emissions by the middle of the century. This would prevent surface temperature anomaly from rising more than 1.5 degrees Celsius above the pre-industrial level, so that the probabilities of these tipping events will be significantly decreased.

This article is based on the paper, "Risk of multiple interacting tipping points should encourage rapid CO2 emission reduction", published in Nature Climate Change, by Yongyang Cai, Timothy M. Lenton and Thomas S. Lontzek.

 

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