This past December, delegates from nearly two hundred countries met in Paris for the latest round of climate talks, the culmination of nearly a decade of meetings with the hope of finally landing on an agreement that would firmly unite the world — in a legally binding way — against worsening climate change. On December 12th, an agreement was reached and in the official document, the Paris Agreement, is the promise that member nations will hold the increase in global average temperature to well below 2°C above pre-industrial levels.
The two degree limit (TDL) has been the official marker of climate catastrophe since 2009, when climate policy experts from around the world met in Copenhagen and sanctioned the limit as the boundary beyond which climate change becomes ‘dangerous’ . The TDL morphed into a guide for future policy changes: to prevent dangerous impacts on the climate system, any future emissions pathway should lead to global temperature rise no greater than two degrees. The question of defining danger, though, or quantifying it scientifically, is wrought with complication, with competing stresses from every angle.
The roots of the TDL go back much further than 2009, to the 60s and 70s when scientists predicted that doubling carbon dioxide concentrations would warm the planet by approximately two degrees . The number stuck, used by economists and policy experts as a clean value around which to revolve analyses and negotiations — but it has yet to be scientifically justified in the modern era of powerful global climate models and more accurate and complete observations.
Reto Knutti, from the Institute for Atmospheric and Climate Science in Zurich, and co-authors recently published a review of the current literature to provide a scientific assessment of the TDL . The studies they mention are performed in a similar fashion: scientists run an array of computer models that simulate the Earth’s climate, each based on the same general physical equations but differing in small ways such as how exactly things like clouds, plants, dust, or mountains contribute to the overall climate. With this toolbox of models in hand, scientists then increase the heat in each model to get a glimpse of various future Earths over a range of temperatures, and examine the resulting behavior of some aspect of the system, such as the precipitation or the sea level rise.
These experiments predict that extreme events worsen with increasing temperature — hot days get hotter, storms more intense, droughts more severe. This is compounded by another result: the number of these extreme events tends to increase nonlinearly, growing exponentially with rising temperature. When averaged over the models, the number of hot days increases 6-fold for a one degree-warmer world, but 20-fold for a two degree-warmer world . General changes in the hydrological cycle — the persistent background climate of a region, canvas to the strokes of storms and droughts — are linked to temperature as well: magnitudes of wetting and drying scale linearly with temperature. Models show that in some places it will rain more, and in some it will rain less. However, in some places the models are unsure , and this uncertainty appears to increase with increasing temperature.
Lack of clarity from model predictions is frustrating for everyone: scientists still don't have an exact idea of what will happen when the planet warms. Within this uncertainty lies the danger of catastrophic climate change. The fraction of land with unknown risks doubles from 20% in a one degree-warmer world to nearly 40% in a two degree-warmer world . No one really knows at what temperature the Greenland ice sheet will reach its breaking point, but scientists are confident it will be abrupt and powerful, accelerating sea level rise in its melt . And what of the other tipping points in the system? When will, after too much heat and drought, the Amazon rainforests dieback, releasing the carbon in their roots. When will the icy methane hydrates found in Arctic permafrost regions melt and release the methane, another greenhouse gas, into the atmosphere?
It is unknown when these thresholds will occur; they could be at the TDL — but what if they occur before? Knutti writes, “In general there is a better understanding of the consequences of crossing such thresholds than where exactly these thresholds may be.” The uncertainties of future climate scenarios, particularly with respect to these thresholds, makes the danger all the more difficult to grasp. What is danger? How many storms or heat waves or meters of sea level rise does it take to realize an unbearable world?
Defining danger becomes an even more excruciating — and scarily existential — exercise, considering we’re in the midst of it now. The situation in much of the world is already dire. Heat waves and droughts, in Russia, the Middle East, and the US, have led to thousands of excess deaths and billions of dollars in wildfire damage . The danger now, in a way, resembles something like a nascent wildfire, with the threat of its chaos and its extension. A majority of coral reef systems will be destroyed once warming surpasses 1.5°C . Both human populations near the equator and small island nations are much more vulnerable to undernutrition, infection, and flood risk for warming above 1.5°C . If global temperature increases by 2°C, Africa heats up 3-3.5°C.
This has led many scientists and policy delegates to criticize the TDL as an outcome of privilege, defining the point of danger as the point when climate change impacts high-latitude developed nations as much as it already impacts poorer, low-lying states. It has created a rift between nations, with a majority of less-developed countries wanting 1.5°C to be the target. And it's led scientists to disregard the TDL as mostly a product of fantasy and convenience.
The truth is, the TDL is only barely achievable. Today, global temperature sits 0.8°C above pre-industrial levels . Two thirds of the carbon dioxide emission quota consistent with the TDL has already been reached . At our current emission rates, it will be reached by mid-century, with an approximately 4°C warming reached by the end of the century . Nearly all climate scenarios modeled by the International Panel on Climate Change (IPCC) predict global temperature will reach the TDL between mid-century and the end of the century; the only exception is the scenario that applies the most stringent emission reductions, requiring carbon to be removed from the atmosphere — an unproven technology .
Knutti et al. concludes that it’s unclear what level of global temperature rise can be considered safe, but the TDL suffices in that we’re much less certain what a two degree-warmer world looks like compared to our current, nearly one degree-warmer version. It seems then that danger is defined primarily by uncertainty, which scientists by nature seek to avoid. So one can understand why, more than anything, they care about action and not whether the limit should be one or two or three degrees. Strangely, that’s the good news. Things are already bad enough that it doesn’t matter when they become worse — as Knutti and others stress, the same procedures are needed to keep temperatures below 1.5°C as are required to keep them below 2°C. What matters, and what should come out of Paris, is that we start now. The TDL has been useful for sparking government cooperation, but a maniacal focus on temperature targets now is unproductive. Now is the time to implement policies that slow, and eventually reverse, the increasing emissions rate. And then we must hope that more and more governments adopt and strengthen those policies — so that the defense of the planet spreads like the fire it seeks to extinguish.
- Sean Faulk (@sean_faulk20)
Signal to Noise Staff Writer
PhD Candidate, Earth, Planetary, and Space Sciences
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