Are temperature targets fit for purpose?

  • 03 Jul 2013, 18:15
  • Freya Roberts

A new study challenges the idea that temperatures targets, such as the widely known 2 degrees Celsius limit, are fit for purpose when it comes to preventing dangerous interference with the climate system.

The research suggests that if other changes in the system, like sea level rise, were factored into climate targets, the amount of carbon we could emit and stay safe would be much lower.

It's a more holistic approach to assessing climate risk - could such an approach benefit other bits of climate research too?

Temperature targets

The most well known climate change target suggests global temperature rise should be limited to two degrees Celsius above pre-industrial times. Whether or not two degrees is the right number, it's been widely touted as a sort of 'safe limit' for well over a decade now and sits at the heart of international climate politics.

Scientists have worked hard to work out exactly what keeping below two degrees would mean for our emissions. In particular, some studies focus on what are called 'cumulative emissions' - for example calculating that if humans release no more than 650 more billion tonnes (gigatonnes) of carbon dioxide by mid-century we will have an 80 per cent chance of staying below two degrees.  

But while these studies focus on how much more carbon is acceptable from a temperature perspective, the new study in the journal Nature suggests safe levels of emissions would be much lower if we factored in other parts of the climate system.

Changes in the system

The study by Steinacher et al suggests that climate targets need to be based on a broader assessment of climate risks, rather than looking just at temperature change. Their modelling shows that emissions consistent with 'safe' temperature rise would produce unacceptable results in other aspects of the climate.

Steinacher and his team took a number of variables related to the climate system - temperature rise, sea level rise, ocean acidification, change in plant productivity, and loss of carbon from soils - and defined what they called acceptable limits of risk. They then used computer models to work out how much carbon dioxide could be released before these limits were reached.

They found that to stay below all of the limits they set, greenhouse gas emissions would have to be much lower than if they were only trying to keep temperature rise below two degrees.

To stand a 66 per cent chance of keeping below two degrees, emissions of carbon would need to be limited to about 570 gigatonnes by the end of the century.  But staying below the safe limits on all six variables, including temperature, would mean limiting emissions to between 290 and 350 gigatonnes of carbon, the model showed.

There are some big uncertainties here, and these figures aren't exact, but the point is fairly clear. When you consider the impact of emissions on other parts of the climate system, cumulative emissions need to be considerably lower.

A holistic approach

The research, published this evening, comes just a few days after a study in the journal Proceedings of the National Academy of Sciences suggested that the combined influence of climate impacts can be much greater than their individual effects.

An international team of scientists modelled how the impacts of climate change on water availability, the spread of disease, agricultural production and ecosystems could overlap in different regions of the world.

Southern Europe, the Ethiopian highlands and northern India emerge as worst-off regions. While individual impacts might affect other countries more severely, the combined effect of multiple impacts make these locations hotspots.

This is preliminary work with some significant uncertainties - in fact it's the first time multiple impacts have been modelled together, the researchers say. But it gives a similar sort of message: looking at changes in one part of the climate system, or one specific impact of climate change, is useful. But looking at the sum of many changes is a much better way to truly gauge what future emissions will mean for climate change, and what the overall impact of those changes will be.

Steinacher et al. (2013). Allowable carbon emissions lowered by multiple climate targets. Nature.  DOI: 10.1038/nature12269

Piontek et al. (2013). Multisectoral climate impact hotspots in a warming world. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1222471110

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