An argument frequently used by those skeptical of the role of anthropogenic greenhouse gases in modern temperature increases is that warming is caused by the Sun.
At first glance, it seems to make intuitive sense: the Sun is a massive nuclear fusion reactor a million times larger than Earth, it is responsible for almost all the energy reaching our plant, and in the past few decades scientists have learned that solar activity varies significantly over time. Surely it must have a larger impact on our changing climate than a gas that comprises only a small fraction of our atmosphere?
However, a detailed analysis of ways in which solar changes could affect Earth’s climate suggests that it is highly unlikely that the Sun is responsible for modern warming. In fact, if the Sun were the major driver of modern climate change, Earth would likely have barely warmed at all over the past 30 years.
As shown in Figure 1 below, prior to the late 1950s greenhouse gases may not have been the dominant cause of climate forcing. According to the study (pdf) this figure is based on, solar was the single largest positive forcing in the early part of the 20th century. When combined with relatively low volcanic activity, solar variations help explain much of the temperature increase at the start of the 1900s. While the importance of solar in climate forcing in the early part of the 20th century has been challenged by some more recent work, virtually all studies indicate that the magnitude of anthropogenic forcings, especially positive forcing from greenhouse gases, has overwhelmed any natural forcing over the past 30 years.
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|Figure 1: Radiative forcing of major climate factors over the past century up until 1995. Figure from Robert Rhode’s Global Warming Art using data from Meehl et al. 2004 (pdf).|
However, those skeptical of the role of carbon dioxide and other greenhouse gases in warming Earth suggest that solar forcings are considerably larger than estimated in the literature. They suggest a number of different mechanisms for a solar influence on Earth’s climate. The most straightforward involves variations in total solar irradiance, TSI, a measure of how much of the Sun’s energy reaches Earth on a given day. Scientists have direct satellite measurements of total TSI since 1979, and proxy reconstructions prior to that.
Comparing TSI to global temperatures reveals that although TSI and temperatures both followed similar trajectories prior to the early 1970s, they have diverged significantly since then, with trends in TSI remaining relatively flat while temperature increases have accelerated.
Figure 2 below shows both TSI and global temperature over the past 127 years. TSI data from 1880 to 2007 is taken from proxy reconstructions by Lean et al. 2004 (text by ftp) and subsequent updates. Smoothed TSI is calculated by an 11 year rolling average to remove the roughly 11 year sunspot cycle, with each year representing the average of its own value, the prior five years and subsequent five years. GISS data is taken from the NASA GISTemp instrumental temperature record and is smoothed using the same procedure.
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|Figure 2 based on data from Lean et al. 2004 (text by ftp) and more recent updates from PMOD composite TSI data and GISS. TSI data is measured in watts per meter squared (W/m^2), while temperatures are measured in terms of anomalies relative to the 1951-1980 mean temperature.|
The Lean study shows the divergence between temperature and TSI over the past 30 years, during which time anthropogenic forcings have unambiguously overwhelmed natural forcings. TSI has increased by a negligible of 0.002 W/m^2 per decade over the past 32 years, while temperatures have increased by 0.18 degrees C per decade over the same period. The hottest year on record in GISS, 2005, and the second hottest year, 2007, both occur near a solar cycle minimum. While decreasing TSI certainly has an effect on global temperature, it is likely overwhelmed by forcing caused by continued increases in atmospheric greenhouse gas concentrations.
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|Figure 3 based on data from Lean et al. 2004 and GISS. TSI data is measured in watts per meter squared (W/m^2), while temperatures are measured in terms of anomalies relative to the 1951-1980 mean temperature.|
An analysis of trends in tropospheric and stratospheric temperatures adds further support to the argument that TSI is not a primary driver of modern warming. If additional energy from the Sun were warming the Earth, one would expect both the stratosphere, the higher part of the Earth’s atmosphere, and the troposphere, the lower part, to warm. If greenhouse gases were the primary cause, however, the troposphere would warm and the stratosphere cool, as the total incoming radiation would remain roughly constant while the amount of energy leaving Earth would decrease as its surface temperature increases.
Satellite measurements from the past three decades show warming in the lower troposphere of about 0.18 degrees C per decade and cooling in the stratosphere of about 0.31 degrees C per decade (see Figure 4). While it is not unambiguously clear that greenhouse gas-driven warming is the primary cause of stratospheric cooling – given that stratospheric temperatures are also affected strongly by ozone depletion, large volcanic events, and other factors – declining stratospheric temperatures do suggest that TSI is not the cause of recent warming.
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|Figure 4: Troposphere and stratosphere anomalies are taken from RSS TLT and TLS band monthly data, respectively, and smoothed by a 12 month rolling average with each month representing the average of its own value, the prior five months and subsequent six months. Large spikes in stratospheric temperature are due to major volcanic eruptions.|
If solar irradiance reaching Earth is not a major driver of modern climate change, could the Sun affect Earth’s climate in other, subtler ways? Skeptics argue that, even if TSI isn’t the culprit, changes in sunspot frequency or the number of cosmic rays striking Earth could be significant drivers of modern warming.
Trends in sunspot counts tend to roughly correlate with TSI, as both are driven by cycles in solar activity. There are few compelling mechanisms that would allow sunspots to influence Earth’s climate other than via changes in solar irradiance, and the lack of a significant trend in sunspots over the past 30 years suggest that they are unlikely to be a major driver of modern warming.
In recent years, a number of studies have questioned a potential link between incoming cosmic rays, high-energy particles generated by various events such as supernova explosions in distant regions of space, and global temperatures. Cosmic rays have been shown (pdf) to increase the number of potential cloud condensation nuculi in the air in controlled laboratory experiments, and are hypothesized to cool Earth by increasing the rate of low-altitude cloud formation.
Cosmic rays are modulated by cycles in solar activity: the stronger the solar activity, the lower the number of cosmic rays that reach Earth. This strong inverse correlation between solar activity and cosmic rays suggests that a cosmic ray-based explanation of modern warming will suffer the same flaws as a TSI-based explanation, namely the absence of a trend.
Figure 5 below shows the number of incoming cosmic rays per minute and the global mean surface temperature over the past 32 years. The data show a slight, though likely insignificant, positive trend in cosmic rays and the same strong 0.18 degree C per decade trend in temperature as before. Given that more cosmic rays are theorized to cool the planet, scientists would expect a strong negative trend over the past three decades if cosmic rays were a major driver of modern warming. The absence of any negative trend in cosmic rays again suggests that they are not responsible for recent temperature increases.
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|Figure 5: Cosmic ray data from the Oulu Neutron Monitor. Temperature data from GISS.|
While a number of solar-related factors are undoubtedly important in shaping Earth’s climate, they seem highly unlikely to have been responsible for the unprecedented rate of warming over recent years. TSI, sunspots, and cosmic rays have lacked any significant trend over the last three decades. Those interested in a more technical analysis of solar trends in recent years, including advanced statistical techniques to filter out cyclical trends in various datasets, are encouraged to read the recent Lockwood and Fröhlich article (pdf).
As for journalists covering climate science, they do well to keep in mind that while a large solar role in modern warming may sound intuitive, it is not supported by actual data. There remain important uncertainties surrounding the role of various solar factors in affecting Earth’s climate, but the consensus in the peer-reviewed scientific literature is that the forcings associated with greenhouse gases have overwhelmed solar and other natural forcings over the past few decades.