2.0 degrees from pre-industrial levels — 1 to 1.5 from todays level — is a ceiling that some world organizations have viewed as troubling, but livable.
Different parts of the globe are impacted differently, raising deep equity issues. Even under higher temperature increases, the dangerous population impacts of climate change fall mainly on the tropics; while temperate zones will see increased risks from storms and heat waves, they may also experience some benefits.
Global average temperature appears to have risen less than one degree centigrade ( 1.9 degrees F) since pre-industrial times, yet polar habitats are warming relatively rapidly, dislocating wildlife and indigenous peoples; subtler changes caused by warming are appearing across the globe. Another degree or two C of warming will exacerbate perennial challenges for large human populations in tropical areas — droughts, flooding, the spread of disease — but may be quite tolerable in the temperate zones of the globe which include the most affluent populations — North America and Europe.
Past 2 degrees centigrade of additional warming, the known consequences and imponderable risks begin to turn negative in most parts of the globe. However, even under rapid temperature rise scenarios, it remains true that the hardest areas are the tropics, especially in Africa and Asia. North American agriculture, for example is projected to slightly benefit through the century, even under the high temperature scenarios. See the fourth IPCC assessment’s working group II report — chapter 14 on North America, at page 631. This report offers region by region assessments of the consequences of warming, mostly under higher temperature ranges — see the Technical Summary at page 32.
Several other recent panel reports also speak to the dangers of different levels of warming. Chapter 3 of the Stern Report offers a systematic assessment of the consequences of different levels of temperature increase. Table 3.1 is a useful summary; in reading this table and considering the consequences of future warming, subtract 0.5 to 1.0 degrees to reflect the fact that this table uses pre-industrial temperatures as its comparison baseline. The Hadley Center Report on Avoiding Dangerous Climate Change compiles studies of likely outcomes of different levels of warming; the steering committee report generally advocates limiting change to 1.5 degrees above current levels. The Hadlley Center also offers a useful overview of the thought process about setting global emissions limits which is more or less recapitulated in the Q&A on the page you are reading. For an argument suggesting 1.0 to 1.5 degrees above current as a ceiling, see the Carbon Equity Project. The European Council has recognized the value limiting growth in temperatures to this level, but apparently doubted the prospect of doing so.
It is worth remembering that the difference between today and the last ice age is only about 5 degrees centigrade. NASA Scientist Jim Hansen has suggested that even 1 degee of additional warming from today is enough to create unacceptable risks from unpredictable ice sheet melting.