57 Responses to NASA’s Hansen: Would recent extreme “events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 ppm?” The “appropriate answer” is “almost certainly not.”
“It is likely that 2012 will reach a record high global temperature.”
Our top climatologist has a must-read, chart-filled analysis, “How Warm Was This Summer?”
The two most fascinating parts are
- Hansen’s discussion of how scientists should answer questions about the recent record-smashing extreme weather events
- Hansen’s analysis of what is coming in the next couple of years.
Let’s start with the extremes:
Finally, a comment on frequently asked questions of the sort: Was global warming the cause of the 2010 heat wave in Moscow, the 2003 heat wave in Europe, the all-time record high temperatures reached in many Asian nations in 2010, the incredible Pakistan flood in 2010? The standard scientist answer is “you cannot blame a specific weather/climate event on global warming.” That answer, to the public, translates as “no”.
However, if the question were posed as “would these events have occurred if atmospheric carbon dioxide had remained at its pre-industrial level of 280 ppm?”, an appropriate answer in that case is “almost certainly not.” That answer, to the public, translates as “yes”, i.e., humans probably bear a responsibility for the extreme event.
In either case, the scientist usually goes on to say something about probabilities and how those are changing because of global warming. But the extended discussion, to much of the public, is chatter. The initial answer is all important.
Although either answer can be defended as “correct”, we suggest that leading with the standard caveat “you cannot blame”¦” is misleading and allows a misinterpretation about the danger of increasing extreme events. Extreme events, by definition, are on the tail of the probability distribution. Events in the tail of the distribution are the ones that change most in frequency of occurrence as the distribution shifts due to global warming.
For example, the “hundred year flood” was once something that you had better be aware of, but it was not very likely soon and you could get reasonably priced insurance. But the probability distribution function does not need to shift very far for the 100-year event to be occurring several times a century, along with a good chance of at least one 500-year event.
I think that is very good advice.
Dr. Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, has gone further, arguing, “It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there’s always an element of both.” Trenberth discussed his perspective in an interview with me:
I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.
Second, you may recall that NASA predicted in January 2009 that 2010 would likely set a record. In this paper, he explains “It is likely that the 2005 and 2010 calendar year means will turn out to be sufficiently close that it will be difficult to say which year was warmer, and results of our analysis may differ from those of other groups. What is clear, though, is that the warmest 12-month period in the GISS analysis was reached in mid-2010, as shown in the Rev. Geophys. preprint.”
Again, this is impressive because we are just coming out of the deepest solar minimum in a century. Hansen provides this figure:
Now Hansen explains why 2012 is likely to set a new record. He notes “Projections of trends over the next few years are possible based on the following considerations”:
- the planet is out of energy balance by at least several tenths of one W/m2 due to the rapid increase of greenhouse gases during the past few decades, as confirmed by measurements of changing ocean heat content
- inertia of energy systems that assures continuing growth of atmospheric CO2 by about 2 ppm per year for the next few years
- expectation that the solar irradiance will climb out of the recent long-lasting solar minimum, as shown in Figure 5
- model projections suggesting that the current La Nina may bottom out near the end of 2010.
He then concludes:
Given the dominant effect of El Nino-La Nina on short-term temperature change and the usual lag of a few months between the Nino index and its effect on global temperature, it is unlikely that 2011 will reach a new global record temperature.
In contrast, it is likely that 2012 will reach a record high global temperature. The principal caveat is that the duration of the current La Nina could stretch an extra year, as some prior La Ninas have (see Nino 3.4 index at the bottom of Figure 3). Given the association of extreme weather and climate events with rising global temperature, the expectation of new record high temperatures in 2012 also suggests that the frequency and magnitude of extreme events could reach a high level in 2012. Extreme events include not only high temperatures, but also indirect effects of a warming atmosphere including the impact of higher temperature on extreme rainfall and droughts. The greater water vapor content of a warmer atmosphere allows larger rainfall anomalies and provides the fuel for stronger storms driven by latent heat.
So 2012 may top 2010 as the hottest year on record with more extreme events. And it’s a presidential election year. If only we had a president who actually talked about climate to the public….