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Book Review

Global temperatures: an uphill battle of warm vs. cold

Article

Dai, A., J.C. Fyfe, S.-P. Xie, and X. Dai (2015), Decadal modulation of global surface temperature by internal climate variability, Nature Clim. Change, 13 April 2015, DOI: 10.1038/NCLIMATE2605

Cover image shows simplified natural warm and cold temperature fluctuations superimposed on a long-term warming trend by Rob Painting from SkepticalScience.com.

Background

Climate models and observations have lately been at a crossroads when it comes to global-mean surface temperatures. State-of-the-art climate models suggest a steady warming trend, while observations show little change in global temperature (Figure 1). This so-called “hiatus” in global warming has been observed since the turn of the twenty-first century and has caused climate skeptics to refute claims of man-made global climate change. Earlier pauses in global warming (e.g. 1940-1949 and 1968-1975) have received considerably less attention. Warming hiatuses have been attributed to decadal cooling in the tropical Pacific, intensifying trade winds, changes in El Nino activity, volcanic activity and reduction in solar heating. It is less understood how much these hiatus periods are regulated by natural climate variability versus external greenhouse gas forcing. This new study identifies the relative contributions of natural climate variability on global mean temperature trends occurring over the past 94 years.

Dai_etal_fig1

Figure 1. Global-mean temperature anomaly shown from observations (black line), model simulations (red line) and models corrected for naturally varying temperature fluctuations (Dai et al., 2015).

Methods

The authors examine observed and model-simulated surface temperatures using data from phase 5 of the Coupled Model Intercomparison Project. The researchers estimate the greenhouse gas forcing on global temperatures and remove the warming trend from the temperature datasets. This procedure helps to isolate the naturally varying component of natural (internal) temperature variability. Additional analyses are run to determine the spatial patterns and amplitudes of temperature fluctuations over the period 1920-2013.

Findings

A natural mode of climate variability called Interdecadal Pacific Oscillation (IPO) accounts for most of the discrepancies between the observed hiatus and modeled warming in surface temperatures. The IPO is a natural oscillation of warm and cold temperatures that alternate every few decades. The current cool phase has been in place since 1999, which coincided with the initial pause in the warming trend. Predominately, the eastern equatorial Pacific began to cool during this time. The temperature footprint of the IPO is not isolated to just the Pacific Ocean, but rather influences global temperatures through ocean heat uptake and air-sea interactions (Figure 2). This study finds that the IPO accounts for 69% of the differences between model-simulated and observed global temperature trends.

Interdecadal Pacific Oscillation (IPO) surface temperature correlation pattern after removing the anthropogenic-warming signal. Warm colors are surface temperatures that are positively correlated to the IPO and cool colors are negatively correlated with the IPO (modified from Dai et al., 2015).

Figure 2. Interdecadal Pacific Oscillation (IPO) surface temperature correlation pattern after removing the human-induced warming signal. When the IPO is positive, warm colors indicated above average temperatures and cool colors represent below average temperatures. During hiatuses, the IPO tends to be negative, which would reverse this pattern (Dai et al., 2015).

In addition to the IPO, the authors find evidence that the Atlantic Multidecadal Oscillation (a natural temperature oscillation in the North Atlantic) has secondary importance when explaining global-mean temperature variability. Since anthropogenic forcing has negligible impacts on both the IPO and Atlantic Multidecadal Oscillation, the authors attribute their variability to unforced natural causes.

Significance

The recent warming hiatus is largely driven by natural climate variability and has some climate change skeptics whimsically thinking that global warming is a hoax. In actuality, natural hiatuses and accelerations in global warming have preferred spatial patterns, with cooling emphasized in the central and eastern Pacific during hiatus periods. These patterns largely differ from those caused by human-induced greenhouse gases and aerosols.

Figure 3. Global mean surface air temperature anomalies overlaid over warm and cool phases of the Interdecadal Pacific Oscillation (England et al. 2014).

Figure 3. Global mean surface air temperature anomalies shown during warm and cool phases of the Interdecadal Pacific Oscillation (England et al. 2014).

Natural variability in the climate system generates periods of both cooling and rapid warming in mean global temperatures (e.g. IPO).  The cool phase of the IPO is associated with warming hiatuses, while the warm phase is associated with accelerated warming (Figure 3). On top of an underlying warming trend forced by greenhouse gas emissions, these natural temperature fluctuations have the potential to mitigate (cool phase) or reinforce (warm phase) the rate of warming. The IPO is difficult to predict; nonetheless scientists speculate that a warm transition and reversal in cooling will occur in the near future. This is likely to cause a sudden uptick in future warming trends with unknown and severe consequences.

 

Your Opinion: After reading about the appreciable influence of internal climate variability on global-mean temperatures, should countries make more of an effort to mitigate their carbon footprint during the warm phases of these temperature oscillations? If so, do you think it would make a substantial difference in long-term warming?

 

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