A restriction to two degrees is still considered as an official climate protection goal. The reason for this: in the event of climate change beyond this level, heat waves, droughts, heavy rains or flooding caused by sea-level rise could become so frequent and so serious that many countries would hardly be able to cope with these climate impacts. In fact, studies show that even two degrees of warming could put some coastal areas in danger, as sea levels are on the rise in South Asia, for example, more than the global average. In addition, the two degrees of the climate protection goal are a global average. Locally, however, temperatures are rising much faster and faster than the global average. Thus, the Arctic has long exceeded the two-degree target. And that's not all. Already in the last World Climate Report of 2014, the Intergovernmental Panel on Climate Change (IPCC) warned that there was not much time left to reach the two-degree target. If total global GHG emissions fell 40 to 70 percent by 2050 compared to 2010 levels, there would still be a 66 percent chance of reaching the two-degree target, according to the IPCC report.
How big is the chance?
We are now three years away - and worldwide CO2 emissions have not dropped significantly since then. Therefore, Adrian Raftery of the University of Washington and his colleagues have now re-examined how realistic the two-degree goal is still. Their model is based on data from the past 50 years on gross domestic product and CO2 emissions per economic unit of all states, as well as the fact that the world's population will grow to 11 billion by 2100, the UN predicts. The national emission targets submitted by the countries for the Paris Climate Agreement have also been taken into account. From this data, researchers determined the likelihood of reaching specific CO2 levels and temperatures by the year 2100.
The results of the model calculations are not very optimistic: According to this, the probability of achieving the 2-degree target set for 2100 is only around five percent. The more ambitious climate protection goal of 1.5 degrees warming is even more unrealistic: its chance is only one percent. "Our analysis shows that the two-degree target is a pretty best-case scenario, " says Raftery. Because the researchers' models argue that mankind will slip significantly past the climate protection target. The chance for warming between two and 4.9 degrees by the end of the century is 90 percent. The median - and thus the likely to be expected means, is 3.2 degrees, as the researchers report. Thus, climate change levels off between the two middle scenarios of the IPCC.
To the scientists' surprise, increasing population growth is unlikely to play a role in this trend. This could be because most of it takes place in Africa - and thus in countries that contribute little to CO2 emissions anyway, as they explain. Instead, the key factor for the climate price is the so-called Carbon Intensity - the amount of CO2 that a country emits per unit of gross domestic product. "Most countries have passed the summit and their carbon intensity is already falling again, " the researchers report. But so far, this trend is not enough. "We need an abrupt change of course in order to reach our climate protection goals, " says Raftery. display
1.3 degrees already inevitable?
Just how much drive climate change has already taken is underlined by a second study published in the same journal. In it, Thorsten Mauritsen of the Max Planck Institute for Meteorology in Hamburg and his colleague examined how the climate develops if now all CO2 emissions worldwide would be stopped immediately. The result: Even with an immediate emission stop, the climate would warm up by 1.3 degrees until the year 2100. The reason: Earth's climate system reacts slowly to changes. It takes around ten years for an increased CO2 concentration in the atmosphere to lead to warming, as a study in 2014 revealed. But this greenhouse effect will last for more than a century - even if all other emissions are stopped.
In extreme cases, this could mean that the climate protection goal of 1.5 degrees may already be exceeded because of this inertia effect - this warming would already be inevitable. "All future CO2 emissions, however, then come on top, " says Mauritsen.
- Adrian Raftery (University of Washington, Seattle) et al., Nature Climate Change, doi: 10.1038 / nclimate3352
- Thorsten Mauritsen (Max Planck Institute for Meteorology, Hamburg) and Robter Pincus (University of Colorado, Boulder), Nature Climate Change, doi: 10.1038 / nclimate3357