Targeting 2 degrees Celcius in Paris, #COP21

The twenty-first session of the Conference of the Parties (COP21) has convened in Paris this week to agree on global solutions to avoid the worst impacts of climate change. The goal is to achieve a legally binding international agreement aimed at reducing greenhouse gas emissions. The last such treaty signed 18 years ago, the Kyoto Protocol, failed to meet many of its objectives since it was not ratified by the US and other developed nations did not fulfill their commitments. In any case, the limits that were put in place in Kyoto run out in 2020, so it is imperative that the world agrees on a sustainable low carbon path for the decades beyond 2020. In Copenhagen in 2009, an attempt to reach a legally binding treaty was unsuccessful, but all of the world’s biggest polluters agreed – for the first time – that deep cuts in the GHG emissions were required. A year later in Cancun, the argument that we must limit the global temperature rise to 2°C above pre-industrial levels to avoid dangerous interference with the climate system took center-stage and has been at the heart of the UN climate conventions ever since.

A conference in July 2015 organized as a prelude to COP21 concluded that “warming to less than 2°C” is “economically feasible” and “cost effective”. Many scientists and economists, however, argue that the 2°C target is overly optimistic or even naive since it requires unrealistically early peaks in global emissions or the rollout of speculative carbon capture technologies (see plot of IPCC emissions scenarios). Once emitted, carbon dioxide stays for thousands of years in the atmosphere. As a result, the 2°C temperature target translates into a very stringent atmospheric CO₂ budget. Given that the current emissions are following the track that is the worst-case emissions scenario (red line in IPCC emissions scenarios), the 2°C target remains only a theoretical possibility.

While the climate negotiations are gaining momentum over the last few months, global climate has crossed some key milestones. The year 2015 is well on its way to being the warmest year on record and the first nine months of the year, the planet was warmer by 1°C above pre-industrial temperatures – halfway to the 2°C target. The atmospheric CO₂ concentrations have also crossed 400 ppm for the first time in over 800,000 years. These figures – 2°C, 400 ppm – do not hold any scientific importance from the point view of climate response, but they serve as easily understood, useful markers to communicate the urgency of the climate change problem. Over 160 countries have submitted their climate pledges ahead of COP21 demonstrating their commitment to tackle climate change. These pledges, however, fall short of a 2°C target and point to a warming of about 2.7°C by 2100 relative to the pre-industrial levels whereas that number is 3.6°C if the world leaders fail to negotiate a deal in Paris.

Global warming estimates for COP21 pledges. Source: Climate Action Tracker

The warming target of 2°C is for global mean temperature. Temperature rise, however, is spatially heterogeneous and land areas in the Northern Hemisphere are warming faster than the Southern Hemisphere. Mean temperature over the US is likely to reach the 2°C threshold a couple of decades earlier than the global mean temperature. This accelerated warming locally and the associated impacts have prompted many states, cities and local governments to take steps to address climate change. Boston, for instance, has a very aggressive Climate Action Plan and is well on its way toward achieving its goals – an 80% reduction relative to 2005 emissions by 2050. Since cities contribute to about 70% of global CO2 emissions, local initiatives such as the one being implemented in Boston, can help reduce global emissions by empowering national governments to set ambitious targets beyond their current pledges.

One of the inevitable consequences of rising temperatures for the next few decades – regardless of decisions made in Paris – is the continued increase in sea level. Even if warming is capped at 2°C, over 100 million people will be affected by rising seas. As a result, the Alliance of Small Island States (AOSIS), have been demanding international measures to limit global warming to 1.5°C. Arguing that the climate change problem was created by developed nations, these countries are also requesting financial help for adaptation measures. How rich countries will help poorer countries – financially and technologically – remains a key issue at COP21. Needless to say, the discussions in Paris will revolve around finances, equity, and climate justice in addition to emissions reductions. Steady progress has been made since Copenhagen in every United Nations climate summit towards an international agreement to be reached in Paris. Limiting the global temperature rise to 2°C may prove challenging; but by working towards that goal, we can slow global warming down and give ourselves more time to mitigate and adapt to changing climate.

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Ambarish is a postdoctoral fellow at the Northeast Climate Science Center at the University of Massachusetts Amherst working with Dr. Raymond Bradley. You can read more about his research here.