Given that incoming radiation from the Sun is one half of the equation that determines the temperature here on Earth, it makes great sense that changes in solar radiation would affect the temperature. And we know this to be the case; when many parts of the Earth experienced cooler temperatures during the 1600s and 1700s, that coincided with a time when not a single sunspot was seen on the Sun’s surface for several decades, implying that solar activity was abnormally low. These days, the Sun is more active, having returned to its normal 11 year cycle where even at the minimum there are usually one or two sunspots around, and at maximum there are loads of them.

So, could solar activity rather than greenhouse gases have driven the recent rise in temperatures? There are two main ways of answering this question: first by directly observing how solar radiation has changed, and secondly by comparing the observed pattern of warming with what you’d expect if the Sun was responsible.

Total Solar Irradiance

The total solar irradiance, or TSI, is the estimated total amount of radiation from the Sun received at the top of Earth’s atmosphere. It can be accurately measured by satellites, and these measurements stretch back to 1979. There are some caveats; different satellites have measured the TSI at different times, and their measurements have to be corrected for the different behaviours of the satellites. Several different approaches have been taken to this, resulting in a few different datasets of TSI since 1978. But essentially, over the period of direct measurement, no matter which dataset is used, the TSI has remained approximately constant.

Further back

Just as there are proxies for temperature which can be used to estimate how the climate has varied over time, so there are with solar activity. The research group at the Max Planck Institute has done a huge amount of work on this. Interestingly, their work suggests that the Sun is more active now than it has been for several thousand years. However, they have found that over the last fifty years, changes in solar activity are not sufficient to drive the observed warming.

Another recent study was carried out by Mike Lockwood and Claus Fröhlich. They found that since the 1980s, all indices of solar activity have been declining, while temperatures have increased. So incoming solar radiation changes and temperatures changes are not well correlated.

Expected effects

You can also consider what would actually happen to the Earth’s climate in response to increasing solar radiation. Given that the Earth is round and that the poles therefore receive a lot less sunlight than the equatorial regions, you’d expect the tropics to warm first. Also, given that sunlight is weaker in winter than it is in summer, you’d expect greater warming in summer than in winter. Finally, given that incident solar radiation is zero at night time, you would expect daytime temperatures to rise much faster than night-time temperatures.

Actually, by each of these measures, the opposite is happening. The most dramatic warming is seen in the Arctic, and on the Antarctic Peninsula. Night time temperatures are rising much faster than daytime temperatures. And winters are getting milder faster than summers are getting hotter. All these things are consistent with increasing absorption of outgoing radiation, and inconsistent with an increase in incoming radiation. The Sun cannot be causing the current warming.

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