The report, Turn Down the Heat: Why a 4°C warmer world must be avoided, warns that an increase of that magnitude "will pose unprecedented risks for humanity". It also argues that the often-implicit assumption that climate change will not undermine economic growth is seriously wrong, and that instead a change in climate will significantly undermine world GDP and poverty alleviation.
And while all countries will be affected, it will hurt developing ones the most where adaptive capacity is at its lowest. Therefore the report says, "The projected 4°C warming simply must not be allowed to occur."
If temperatures do hit 4°C the consequences will be devastating, it says. These range from inundation of coastal cities, to increasing risks for food production potential, to substantially exacerbated water scarcity in many regions.
"And most importantly, a 4°C world is so different from the current one that it comes with high uncertainty and new risks that threaten our ability to anticipate and plan for future adaptation needs."
Importantly, it says, "The science is unequivocal that humans are the cause of global warming, and major changes are already being observed."
Even if countries meet the current pledges on the table in the United Nations Framework Convention on Climate Change discussions, currently underway in Doha, Qatar, the temperature will hit 4°C. But the underpinning of many of these negotiations is that a maximum 2°C change is allowable.
The key findings are:
- Average global temperatures are 0.8°C up from pre-industrial levels.
- Absolute temperatures will increase the most in higher altitudes, but warming in the tropics will push average temperature to extremes which ecosystems have not adapted to.
- Sea levels could rise by up to a metre, with the greatest rise around the tropics.
- A disproportionate increase in tropical cyclones in low-altitude regions.
- The concentration of greenhouse gases in the atmosphere has continued to rise, from the preindustrial concentration of 287 parts per million to over 391 in September, with a rise of 1.8 parts per million every year at present. The COP negotiations are aiming to cap this rise at 450, where it is believed temperature increases can be reined-in at 2°C.
- Carbon dioxide will also dissolve in the ocean at increased rates, resulting in acidification.
This is particularly damaging for coral reefs.
These kinds of rapid changes are evident in the concentration of large-scale environmental problems around the world. An example is the recent drought in the United States, which impacted 80% of agricultural land and was the worst since the 1950s.
The 2010 heat wave in Russia killed 55 000 people, destroyed 25% of cropland and led to a 1% loss of GDP.
And as many ecosystems reach their tipping point – where many smaller changes lead to the system changing totally – the damage to these systems will dramatically reduce their ability to provide ecosystem services. This will have a huge effect on what humans can get from nature, it says.
Sub-Saharan Africa is identified in the report as particularly vulnerable. "It is an example of an environment where impacts across sectors may interact in complex ways with one another, producing potentially cascading effects that are largely unpredictable," it says.
And the biggest immediate problem for humans will be the impact on agriculture, which will be heavily affected by droughts and a change in local ecosystems (75% of the region's agriculture is rain-fed). "These conditions are expected to increase the scale of population displacement and the likelihood of conflict as resources become more scarce," the report says.
With this scenario seeming all the more likely as talks stall, the report says it will be people in these developing regions that will lose the most in a world where average temperatures rise by more than 4°C.
Levels of greenhouse gases therefore have to be kept to a maximum 450 parts per million. "Only early, cooperative, international actions can make that happen," it concludes.