26 October 2012

NASA climate chief demolishes denialist claims on sea levels

NASA climate chief James Hansen has three great attributes: his research is impeccable and pertinent, he is politically courageous, and he communicates with great clarity to lay audiences.
     Reading his presentation on 5 September — when he stepped in the deniers’ lion’s den at a meeting organised by the climate-denialist lobbyist Grover Norquist to rebut arguments previously presented by Pat Michaels — all three features of Hansen’s work were on display.
     Hansen shows how Michaels uses the denials’ favourite method – cherry-picking data over a particular range – to prove that black is white.
     Last week in Deniers cherry-pick in vain effort to prove up is down, we looked at how that technique had been used to falsely claim that “Global warming stopped 16 years ago”.
     In his 5 September presentation, Hansen focuses on the assertion that sea-level rises have slowed/stopped. The real issue here, as with short-term temperature movements, is the influence of the El Nino–La Nina cycle. During the La Nina phase, there is more atmospheric water vapour, there is more precipitation and hence more water stored on land. All of which means that there is a bit less water stored in the oceans. So during a strong La Nina, sea levels will temporarily flatten or drop a little, and then rise strongly with the advent of the next El Nino phase.
     But behind all this short-term fluctuation, the longer-term trend is rising sea levels.  This can be seen clearly in this first chart, from Jim Hansen. The bottom half shows the ENSO index, with El Nino phases in orange and La Nina phases in blue. From 2010 to early 2012, we can see a strong La Nina which brought some record heavy downpours to Australia.
In the top part of the chart, actual data (in blue) is smoothed to a 1-year running curve (red) which shows a drop in sea levels in 2010 associated with the strong La Nina, and then rising strongly to be above the long-time linear fit (black line) by mid-2012 as the La Nina ended and conditions returned to neutral.
     Hansen notes that this latest data was available to Michaels when he gave his presentation, but he chose not to include it for the obvious reason that it contradicted his denialist, cherry-picking  interpretation. Hansen concludes:
A simple honest presentation of all the data, as in our Fig. 1a, would have made clear that the rate of sea level rise is not declining. A professor likely would give Michaels an "F" for his analysis of the rate of sea level rise. If the professor were Machiavelli, perhaps the grade would be elevated, but not much.
Hansen also presented a chart on the longer-term sea-level-rise data, showing that:
The average in the 20th century rise was about 2 mm/year. In the last two decades the average rate of rise has been 3.1 mm/year. The recent rate of sea level rise corresponds to 3.1 meters per millennium, which is at least an order of magnitude faster than sea level rise during the past 5000 years of the Holocene, when sea level rose by only 1-2 m.
 The sea-level rise discussion concludes with a reiteration of observations and predictions that Hansen has been making for many years:
  • There is uncertainty and disagreement within the scientific community concerning how rapidly the rate will grow, but not on the fact that “continued global warming is likely to cause the rate of sea level rise to increase, and that burning all the fossil fuels would cause eventual sea level rise of many meters perhaps tens of meters”.  Because of its potential importance, the topic of “how fast” warrants discussion.
  • Multi-meter sea level rise is likely this century if humanity follows a "business-as-usual" greenhouse gas emissions path. If the present fossil fuel emissions path continues, the climate forcing (energy imbalance) will exceed any known forcing in the paleo-climate record. That record includes instances in which warming climate caused sea-level rise as great as 4-5 meters in a century.
  • The West Antarctic ice sheet is the ice sheet most vulnerable to possible collapse. It rests on bedrock well below sea level, so is potentially capable of rapid escape to the ocean, raising sea level several meters.
  • The opinions of Hansen and his colleagues on the near-term vulnerability of the ice sheets is at the high end of what the scientific community has discussed. The slow pace at which the scientific community has moved its estimates of future sea level rise to higher values could be described as "scientific reticence".
In the remainder of his presentation, Hansen present evidence on surface temperatures, the performance of climate models, and key issues for today. The whole presentation is only 9 pages, and it’s well worth the read.