Most CP readers know about the 2°C warming limit, but many don’t appreciate its full implications. This short essay by two of the analysts who completed the first comprehensive analysis of that limit back in 1989 elaborates on the most important of these implications. Author bios and all references are at the end. Koomey has been a friend and colleague for more than a decade and a half. The figure comes from MetroNaturel.
Why two degrees really matters
Jonathan G. Koomey and Florentin Krause[i]
When the countries of the world meet for climate negotiations in Copenhagen this month, they will discuss how to prevent global temperatures from increasing more than 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels. This warming limit, accepted in principle by the leaders of the G8 countries in July,[ii] is more than just a number””it represents a way to think about the climate problem that can help us develop and evaluate options for solving it.
The current trajectory for greenhouse gas emissions would move the Earth by the middle of this century well outside the temperature range in which humanity evolved, marked by the 2-degree limit. This trend increases substantially the risk of dangerous, irreversible, and, perhaps, catastrophic changes in the global life support systems upon which we all depend.[iii] As the White House Science Advisor John Holdren aptly puts it, we’re “driving in a car with bad brakes towards a cliff in the fog.”[iv] The 2-degree limit is like a road sign warning us to avoid the cliff ahead.
Defining a warming limit implies a greenhouse gas budget, which is an upper limit to our cumulative emissions over the next 50 to 100 years. Such a budget encapsulates our scientific understanding of how emissions interact with the Earth’s climate and affect global temperatures. Some of the most significant greenhouse gases, like carbon dioxide, stay in the atmosphere for many decades,[v] which is why the budget is defined over the long term.