The long-term trend of rising global mean surface temperature over the 20th and 21st centuries has been overlain with variability, including short periods of cooling or slower warming, and periods of more rapid warming. The most recent slowdown, in the early 2000s, attracted widespread media attention and has been an area of considerable scientific interest. Analysis of rolling 15-year trends by the Met Office shows that the rate of warming slowed between 1999 and 2014. We now understand many aspects of the causes of this slowdown, but further research is ongoing using a range of different techniques.

A recent paper published in Science Advances by Prof Chris Folland and co-authors adds to the understanding of the causes of variability in global mean surface temperature from 1891 to 2015. Consistent with other recent Met Office research, this work confirms that decadal variability in the Pacific Ocean (the Interdecadal Pacific Oscillation, IPO, also referred to as the Pacific Decadal Oscillation, PDO) was a major factor that masked the long-term trend of anthropogenic warming for a short time during the early 21st century.

The paper also uses an updated estimate of solar forcing (Matthes et al, 2017), which will be used internationally in future climate modelling studies (CMIP6), and an updated estimate of volcanic forcing. Three dominant factors are shown to be important in driving the most recent slowdown: a negative (cooling) phase of the IPO, reduced solar irradiance, and a smaller cooling effect from volcanic eruptions.

The relative importance of the IPO and solar forcing depends on the time period that defines the slowdown. Over the period of 1998-2013, IPO forcing is dominant, whilst solar forcing has a greater influence over the sub-period between 2003 and 2011.

Previous studies have identified Pacific climate oscillations, solar, and volcanic forcing as important components of the early 20th century slowdown in warming. This study by Folland et al. provides further evidence that this combination of factors temporarily countered much of the warming influence from anthropogenic greenhouse gases and aerosols during this time.

In recent years, the slowdown period was confirmed to have ended following a series of record-breaking global mean surface temperatures since 2014, each exceeding pre-industrial temperatures by more than 1 °C.

Does this change our understanding of the slowdown?

This study provides another estimate of the role that the combined influences of the IPO, solar and volcanic forcing played in the recent slowdown. Compared to earlier studies, this work uses forcing data that have been updated. However, the uncertainty surrounding these forcing estimates means that while we can be confident that a combination of these factors can explain the slowdown, it is not possible to pin down a single dominant cause or to rank which factor is most important:

A minimum in solar irradiance was reached in 2010, and this updated dataset supports previous findings that solar forcing played a cooling role over this time period (Schmidt et al. 2014, Huber and Knutti 2014, Kaufmann et al 2011).

The switch to a negative phase of the IPO, likely to have been enhanced by regional anthropogenic aerosol forcing, is found to be a major factor in the slowdown, consistent with previous studies (Meehl et al 2011, Kosaka and Xie 2013, England et al 2014, Trenberth et al 2014, Smith et al. 2016).

Additionally, this study confirms that volcanic forcing made a small contribution to the slowdown (Santer et al. 2014, Schmidt et al. 2014, Huber and Knutti 2014, Haywood et al 2014, Fyfe et al 2013, Solomon et al. 2011).

As with previous studies, this work also concludes that anthropogenic forcing is by far the largest factor in long-term climate change, consistent with the scientific consensus that emissions of greenhouse gases from human activities are extremely likely to have been the dominant cause of warming since the mid-20th century (IPCC AR5).

For a link to the paper, click here. Folland, C.K., O. Boucher, A. Colman and D.E. Parker, 2018: Causes of irregularities in trends of global mean surface temperature since the late 19th century. Science Advances,. 4, eaao5297 (2018).

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