Sun & climate: moving in opposite directions

What The Science Says:
In the last 35 years of global warming, the sun has shown a slight cooling trend. Sun and climate have been going in opposite directions.

Climate Myth: It's the sun
"Over the past few hundred years, there has been a steady increase in the numbers of sunspots, at the time when the Earth has been getting warmer. The data suggests solar activity is influencing the global climate causing the world to get warmer." (BBC)

As supplier of almost all the energy in Earth's climate, the sun has a strong influence on climate. A comparison of sun and climate over the past 1150 years found temperatures closely match solar activity (Usoskin 2005). However, after 1975, temperatures rose while solar activity showed little to no long-term trend. This led the study to conclude, "...during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source."

In fact, a number of independent measurements of solar activity indicate the sun has shown a slight cooling trend since 1960, over the same period that global temperatures have been warming. Over the last 35 years of global warming, sun and climate have been moving in opposite directions. An analysis of solar trends concluded that the sun has actually contributed a slight cooling influence in recent decades (Lockwood 2008).

TSI vs. T
Figure 1: Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance (thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Krivova et al 2007 (data). TSI from 1979 to 2015 from PMOD (see the PMOD index page for data updates).

Foster and Rahmstorf (2011) used multiple linear regression to quantify and remove the effects of the El Niño Southern Oscillation (ENSO) and solar and volcanic activity from the surface and lower troposphere temperature data.  They found that from 1979 to 2010, solar activity had a very slight cooling effect of between -0.014 and -0.023°C per decade, depending on the data set (Table 1, Figure 2).

Table 1: Trends in  °C/decade of the signal components due to MEI, AOD and TSI in the regression of global temperature, for each of the five temperature records from 1979 to 2010.

table 3

Figure 7

Figure 2: Influence of exogenous factors on global temperature for GISS (blue) and RSS data (red). (a) MEI; (b) AOD; (c) TSI.

Like Foster and Rahmstorf, Lean and Rind (2008) performed a multiple linear regression on the temperature data, and found that while solar activity can account for about 11% of the global warming from 1889 to 2006, it can only account for 1.6% of the warming from 1955 to 2005, and had a slight cooling effect (-0.004°C per decade) from 1979 to 2005.

A number of studies have used a variety of statistical and physical approaches to determine the contribution of greenhouse gases and other effects to the observed global warming.  Those studies find a relatively small solar contribution to global warming, particularly in recent decades (Figure 3).

solar attribution

Figure 3: Solar contribution to global warming according to various peer-reviewed attribution studies.  The studies are Meehl et al. 2004 (M04, blue), Stone et al. 2007 (S07, red), Lean and Rind 2008 (LR08, green), and Huber and Knutti 2011 (HK11, purple).

Other studies on solar influence on climate

This conclusion is confirmed by many studies finding that while the sun contributed to warming in the early 20th Century, it has had little contribution (most likely negative) in the last few decades:

Intermediate rebuttal written by Larry M

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Additional video from the MOOC

Expert interview with Mike Lockwood

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