I know that Sou has already discussed this, but I thought I would add a comment of my own. Bob Tisdale has a new post on Watts Up With That (WUWT) called another peer-reviewed paper predicting the cessation of global warming will last at least another decade. The post is about a new paper by Li et al. (2013) which suggests that the current “hiatus” may last another decade or so.
Basically the paper decomposes the northern hemisphere temperature record into two parts : a long-term trend (which is rising) and an oscillation. They determine that the oscillatory part correlates well with the North Atlantic Oscillation (NAO – the dominant mode of atmospheric variability over the North Atlantic region), with a lag of 16 years. Given that there is a lag of 16 years, they assume that they can predict the future northern hemisphere temperature using the known NAO values. However, they do have to make an assumption with respect to the long-term trend, which they assume is linear. The function they use to predict future northern hemisphere temperatures is below.
So, they use the NAO values (which is simply an oscillatory function) and assume that the long-term trend is linear in time. The coefficients a, b, and c are determined via linear regression and are a = 0.38, b = 0.08, and c = 16 (they are determined using two different past time intervals, but these are approximately the values). This means that this model assumes an underlying warming trend of 0.08oC per decade.
Between 1990 and 2011, the NAO decreased by 0.5, so the first term in the above equation (given a = 0.38) means the the NAO would contribute to a drop in NH temperatures of 0.19oC between 2011 and 2027. The linear trend part of the above equation would imply that between 2007 and 2027, temperatures should have risen by 0.16oC. Hence, the Li et al. (2013) model suggests a small drop in NH temperature between now and 2027. However, what Bob Tisdale fails to acknowledge, is that this still includes an underlying warming trend of 0.08oC per decade. So, this does not – in any way – contradict mainstream climate science. Furthermore, the NAO appears to vary by about 0.8-1 over a period of 40-60 years. After 2027, when the NAO is likely to start producing a rise in temperatures, the equation above would indicate that NH temperatures would likely rise at about 0.2-0.3oC per decade (assuming that the model has any merit).
So, a few closing remarks.
- To a certain extent, this is just a curve fitting exercise. Having said that, it doesn’t seem unreasonable that oscillations in the temperature record correlate with things like the NAO.
- It only applies to the Northern Hemisphere, so am not sure what it would predict for global temperatures.
- The model fundamentally relies on an assumption that there is a long-term, anthropogenic warming trend. Therefore, what’s presented here in no way contradicts the underlying theory of anthropogenic global warming.
- Although the model “knows” the NAO index relevant for the period up until 2027, it does not know the underlying warming trend and so has to assume that this will continue with the same trend as it has in the past. Given that anthropogenic forcings are getting ever more significant, this assumption may not be well-founded.
- If we assume that CO2 concentrations will double – relative to pre-industrial times – by about 2050, the underlying warming trend used here (0.08oC per decade) would imply a transient climate response (TCR) of about 1.4oC, entirely consistent with many other estimates.
- The NAO appears to be an oscillation with a period of about 60 years and with an amplitude of about 1. Hence a flat trend for the next few decades (because the NAO influence cancels the underlying warming trned) then implies a period after that where the temperature will rise at least twice as fast as the underlying warming trend.
- There is, though, a fundamental problem with the temperature trend remaining flat or falling slightly between now and the mid 2020s. Today, we’re accruing energy at about 1022J per year, most of which is going into the oceans and a small fraction is contributing to a slow (0.05oC per decade) rise in surface temperatures. In 20 years time, if the temperatures remain flat, the added forcings due to increased atmospheric CO2 concentrations would have at least doubled this energy imbalance. For temperatures to remain flat, an ever increasing fraction of this would have to go into the oceans. This may be possible, but would seem unlikely.
- So, an interesting paper but one that doesn’t contradict any of the fundamentals of AGW and that probably suffers from being rather simplistic when it comes to making predictions about the next 16 years. It’s possible that trends could remain flat into the 2020s, but I suspect that this is rather unlikely.