New Paper: On Forced Temperature Changes, Internal Variability and the AMO

Michael Mann, Byron Steinman, and Sonya Miller have just put out a new paper on climate change which addresses a number of key concerns. The paper is called “On Forced Temperature Changes, Internal Variability and the AMO.” Here’s the abstract:

We estimate the low-frequency internal variability of Northern Hemisphere (NH) mean temperature using observed temperature variations, which include both forced and internal variability components, and several alternative model simulations of the (natural + anthropogenic) forced component alone. We then generate an ensemble of alternative historical temperature histories based on the statistics of the estimated internal variability. Using this ensemble, we show, firstly, that recent NH mean temperatures fall within the range of expected multidecadal variability. Using the synthetic temperature histories, we also show that certain procedures used in past studies to estimate internal variability, and in particular, an internal multidecadal oscillation termed the “Atlantic Multidecadal Oscillation” or “AMO”, fail to isolate the true internal variability when it is a priori known. Such procedures yield an AMO signal with an inflated amplitude and biased phase, attributing some of the recent NH mean temperature rise to the AMO. The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming. Claims of multidecadal “stadium wave” patterns of variation across multiple climate indices are also shown to likely be an artifact of this flawed procedure for isolating putative climate oscillations.

The key points of this paper, which I cribbed directly (with minor modifications) from Michael Mann’s Twitter stream, are:

• Warming of the past decade, during which time some have claimed global warming to have experienced a “pause,” is within expected range given internal variability.
• Certain common procedures fail to isolate internal variability in climate.
• The Atlantic Multidecadal Oscillation (AMO) appears to have been in a cooling phase in recent decades.
• “Stadium wave” patterns appear to be methodological artifacts of flawed assessment procedures.

So the pause is looking increasingly like a faux pause. The relationship between large scale decade-level variations in climate systems to long term warming is better understood. And, very interestingly, a previously proposed method of explaining the so-called “pause” was found wanting.

The “Stadium Wave” model found a signal in the data that appears to arise from the AMO and propagate across a number of climate subsystems and seemed to explain a pause in global warming, further suggesting that this pause may last until 2030 or so. When models were run by Mann et al that were explicitly designed to not include the necessary properties to develop a “stadium wave” they seemed to have this property anyway, which was further amplified by the procedure used to “detrend” (eliminate the long term effects of global climate change, leaving behind decade-level variation) were applied to the data. The “stadium wave” effect seems to have arisen initially from interaction of essentially random variables in the procedure and was then further accentuated by the detrending method. Putting it a slightly different way, the meaningful part of the long term climate signal, warming and other known factors, explains the climate signal best and the “stadium wave” is an artifact of an untried and untested method.