What's So Special About 2 Degrees Celsius?

12/02/2015 03:35 EST | Updated 12/02/2016 05:12 EST
C. Allan Morgan via Getty Images
Temperature inversion (smog) over Tucson, Arizona

With the 21st United Nations Convention on Climate Change (COP21) in the news we keep hearing about the need to keep global climate change below a target of two degrees Celsius. However, few people know where this two degrees target comes from. The reason for this is that the two degrees Celsius target is one of the most deliberately muddied topics in the climate change debate. Why would this be? Because this particular target is not a scientific number, but rather a political one.

The bulk of the modern research indicates that the most likely range for strongly negative results derived from climate change is somewhere between 1.5 and four degrees Celsius. The reason we retain the two degrees target is that unlike a truly science-based target, it has not been refined as our knowledge base has improved.

So, where did the two degrees target come from? At the 1992 United Nations Framework Convention on Climate Change (UNFCCC) it was agreed that there was a need to stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.

The problem is that no one knew what it would take to "prevent dangerous anthropogenic interference in the climate." At that point in time the field of climate change was still in its infancy. The first integrated assessment global climate models (GCMs) were still being developed and the computer power needed to do the complex calculations was not readily available.

Consider that an iPad 2 would have made the list of the world's speediest supercomputers until 1994 -- as such, the GCMs were necessarily basic. In the absence of modern computational power, a number of scientists and governments went to work trying to come up with a number.

As described on RealClimate blog, the critical assessment ended up being from the German government's Advisory Council on Global Change. Lacking the quantitative tools, they took a qualitative look at the problem. In doing so they identified a "tolerable temperature window" based on historic reconstructions of temperature regimes and came up with the following argument:

This geological epoch has shaped our present-day environment, with the lowest temperatures occurring in the last ice age (mean minimum around 10.4 degrees Celsius) and the highest temperatures during the last interglacial period (mean maximum around 16.1 degrees Celsius). If this temperature range is exceeded in either direction, dramatic changes in the composition and function of today's ecosystems can be expected. If we extend the tolerance range by a further 0.5 degrees Celsius at either end, then the tolerable temperature window extends from 9.9 degrees Celsius to 16.6 degrees Celsius. Today's global mean temperature is around 15.3 degrees Celsius, which means that the temperature span to the tolerable maximum is currently only 1.3 degrees Celsius.

So, starting with the approximate 0.7 degrees that had already been observed by 1995 and adding their estimate of 1.3 degrees, they ended up with a number of two... and the rest was history. The German position was adopted by the European Union and it became the de facto target we know and love. In essence, the two degrees Celsius target is a sensible-sounding qualitative number based on approximate temperature ranges for interglacial periods, buffered with a fudge factor.

So, is there anything wrong with this target? Two obvious questions are:

1) Is it the right number? and

2) Does it makes sense to use a lagging indicator as your target?

Let's deal with the second issue first. To explain, temperature is a lagging indicator in that temperature only rises after carbon dioxide concentrations have risen. Moreover, as demonstrated by "the pause", temperature can be a delayed lagging indicator since global temperature has essentially held still (or increased marginally) for about 20 years while carbon emissions have continued to grow. By depending on a lagging indicator we will only be sure we have reached the edge of the chasm after we have gone off the cliff -- the Wile E. Coyote approach to climate policy.

If one truly believes that two degrees Celsius represents a danger point, then it would be much better to establish what emissions will get you to that point and ensure we do not exceed those numbers. This is the approach suggested by Meinshausen in Nature and represents the current approach at COP21 of setting carbon budgets.

Another consideration in the debate is whether two degrees even represents an appropriate target in the first place. Recent research indicates that small amounts of heating (somewhere between 0.5 degrees and 1.5 degrees) may actually result in improvements in global quality of life. The positive effects of small increases in atmospheric carbon dioxide concentrations are undeniable. Plants grow better and have more drought tolerance under conditions of higher atmospheric carbon doxide concentrations and a slightly warmer world ends up being better for many ecological communities (including humans).

The problem is the exact point (of temperature increase/ CO2concentrations) at which things go downhill remains unclear, as well as the global CO2 concentrations necessary to elicit that warming.

Most problematically, the observed improvements in quality of life derived from the first one degree Celsius of warming might actually be hindering our attempts to slow the growth in carbon emissions. Many ask: why change when all is going so well?

To attempt to change the narrative activists have attempted to link higher carbon dioxide concentrations to wild weather and unexpected global events. The problem is the peer-reviewed research and the Intergovernmental Panel on Climate Change say that these links are not scientifically defensible, which only feeds the appetite against action.

Perhaps the activists out there might want to acknowledge the science which says that increased CO2 concentrations are actually expected to improve things in the short run. Framing the current conditions as a "calm before the storm" would better allow the public to understand the risk. In doing so the activists may wish to remember what happened to the shepherd boy when the real wolf finally came to call.


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