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The recent slowdown in the warming rate of the Northern Hemisphere may be a result of internal variability of the Atlantic Multidecadal Oscillation a natural phenomenon related to sea surface temperatures, according to a new paper from Michael Mann and others just published in Geophysical Research Letters.
Mann, from Pennsylvania State University in the US, explained the research in a recent news release which we reported here. "Some researchers have in the past attributed a portion of Northern Hemispheric warming to a warm phase of the AMO," he said. "The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming temporarily."
"We conclude that the AMO played at least a modest role in the apparent slowing of warming during the past decade," said Mann. "As the AMO is an oscillation, this cooling effect is likely fleeting, and when it reverses, the rate of warming increases." This is critical for the debate about the sensitivity of the climate to carbon dioxide increases and estimates of what is called the equilibrium climate sensitivity (ECS). Mann and his co-authors, Byron Steinman and Sonya Miller, write in their paper that their results argue “against the notion that the slower rate of warming over the past decade requires... any lowering of canonical ECS estimates.”
According to Mann, the problem with the earlier estimates of the influence of the AMO stems from having defined the AMO as the low frequency component that is left after statistically accounting for the long-term temperature trends, referred to as detrending. Initial investigations into the multidecadal climate oscillation in the North Atlantic were hampered by the short length of the instrumental climate record which was only about a century long.
Mann and his colleagues took a different approach in defining the AMO. They compared observed temperature variation with a variety of historic model simulations to create a model for internal variability of the AMO that minimizes the influence of external forcing including greenhouse gases and aerosols. They call this the differenced-AMO because the internal variability comes from the difference between observations and the models' estimates of the forced component of North Atlantic temperature change. They found that their results for the most recent decade fall within expected multidecadal variability.
They also constructed plausible synthetic Northern Hemispheric mean temperature histories against which to test the differenced-AMO approaches. Because the researchers know the true AMO signal for their synthetic data from the beginning, they could demonstrate that the differenced-AMO approach yielded the correct signal. They also tested the detrended-AMO approach and found that it did not come up with the known internal variability.
The detrended approach produced an AMO signal with increased amplitude, both high and low peaks were larger than in the differenced-AMO signal and in the synthetic data. They also found that the peaks and troughs of the oscillation were skewed using the detrending approach, causing the maximums and minimums to occur at different times than in the differenced-AMO results. While the detrended-AMO approach produces a spurious temperature increase in recent decades, the differenced approach instead shows a warm peak in the 1990s and a steady cooling since.
The authors write in their paper: “Applying the Detrended-AMO approach directly to the model-estimated forced component alone, we observe these same main features, including the 1950s-1970s decrease... We conclude that those features arise from forced changes in temperature rather than internal multidecadal variability”.
Past researchers have consequently attributed too much of the recent North Atlantic warming to the AMO and too little to the forced hemispheric warming, according to the scientists.
Mann and his team also looked at supposed "stadium waves" suggested by some researchers to explain recent climate trends. “Our analysis suggests that this feature is instead an artefact of the residual forced signal that remains after linear detrending (the Detrended-AMO procedure), combined with random perturbations in the apparent phase of the “oscillation” for any particular climate index, due to the low-frequency effects of the additive noise,” they write in their paper.
The paper concludes that “there is no inconsistency between recent observed and modeled temperature trends. As a corollary, recent temperature observations are entirely consistent with prevailing mid-range estimates of climate sensitivity.”
The conclusion continues: “We use the same ensemble to evaluate the faithfulness of the “Detrended Residual” approach to estimating internal (AMO-related) variability, wherein temperature data are linearly detrended, the residual is interpreted as internal variability, and the multidecadal component of the residual is interpreted as representing a low-frequency “AMO” oscillation. In cases where the signal is known a priori, we show that this procedure yields a biased estimate of the true AMO signal in the data. The procedure attributes too large an amplitude to the AMO signal and a biased estimate of its phase. Wherein application of the flawed Detrended-AMO approach attributes some of the recent NH mean temperature rise to an AMO signal, the true AMO signal instead appears likely to have contributed to a relative cooling over the past decade, explaining some of the observed slowing of warming during that timeframe. We find that claims of a “stadium wave” AMO signal propagating through the global climate are likely an artefact of the Detrended-AMO procedure as well.”
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, first, 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.
On forced temperature changes, internal variability, and the AMO by Michael E. Mann, Byron A. Steinman and Sonya K. Miller. Article first published online: 1 MAY 2014 in Geophysical Research Letters. DOI: 10.1002/2014GL059233
Read the abstract and get the paper here.
Quotes from the paper and images courtesy of Geophysical Research Letters, American Geophysical Union and the authors. An earlier report from Penn State on this research which we published here based on a news release from Penn State here.
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