The Earth receives about 2 x 10E17 Watts from the Sun. It is the energy that keeps the Earth habitable and drives the weather and climate system. How do variations in the Sun’s output affect the Earth is a persistent question in climate science.
The answer depends upon how you look at it. Many say not very much. Something of the order of 0.3° C over decades is often cited and compared to the 0.2° C per decade some claim has already taken place.
When the solar wind strikes the Earth.
On the face of it that means the Sun is important but minor in that a decade and a half of greenhouse gas forced warming will see it overtaken. However, looking back on the past 20 years there hasn’t been anything like 0.4° C of global warming (outside short-term El Ninos) so from today’s standpoint 0.3° C seems a much bigger deal.
Some predict little warming in the decades due to come because of a conspiracy of natural factors, including the Sun, offsetting what is expected by the IPCC.
Others are more categorical linking the Little Ice Age – a prolonged period of cool global temperatures in medieval times, thought to be confined to Europe but now shown to be world-wide – to changes in the Sun’s behaviour. During the so-called Maunder Minimum (approx. 1620 to 1720) it’s believed Solar output declined slightly and sunspots were certainly absent. Currently the Sun might be entering another Maunder-like state that may have the same effect on global temperature.
As usual Sun-Earth connections are not straightforward. It has been pointed out that the Little Ice Age lasted centuries inside which the Maunder Minimum was a relatively short period. A new study brings together models and observations. Using observations from the northern hemisphere alone researchers suggest that the Little Ice Age spanned about 480 years from 1440 – 1920 suggesting, they say, that there is no cause and effect between the Maunder Minimum and the Little Ice Age. They believe that multiple factors such as volcanic activity, caused the Little Ice Age and that the Sun’s declining output had an effect on a par with changing land use. Having the Little Ice Age end around 1920 is problematical for those who believe that all of the Earth global temperature increase over the last century is anthropogenic. Did human influences alone end the Little Ice Age?
Solar physicists often take a different view than climate scientists. They are often of the belief that solar effects are important on the regional scale. Quite a lot of regions actually. But how else could the Sun affect the climate if not by changes in its radiative output which are held to be to small for a global effect?
A New Climate Driver
Well-known is the Svensmark theory that the Sun’s magnetic influence can expose or protect the Earth to cloud-seeding cosmic rays. But there could be other effects coming from the Sun’s non-light output. A study suggesting that the Sun’s behaviour could affect northern winters may provide more evidence that not only solar electromagnetic radiation but also the solar wind can affect the climate. It’s possible because the solar wind interacts with the magnetosphere and directs its energetic particles into the inner magnetosphere. This happens more often, and to a greater effect, during the declining phase of the 11-year solar cycle during which high-speed solar wind streams are more commonly detected at Earth.
Electrons penetrate the mesosphere and upper stratosphere, where they can create nitrogen and hydrogen oxides. During winter nitrogen oxides can descend down to the mid-stratosphere and destroy ozone, which leads to cooling of the high-latitude stratosphere. This enhances temperature gradients and westerly winds accelerating the polar vortex.
According to the researchers, “In addition during a strong polar vortex, the North Atlantic Oscillation (NAO) is anomalously positive. Positive NAO encloses the cold arctic air into the polar region and enhances the westerly winds at mid-latitudes. Enhancement of westerlies bring warm and moist air from Atlantic to the Northern Eurasia causing positive temperature anomalies. At the same time negative temperature anomalies are observed in the Northern Canada and Greenland.” Hence the solar wind can affect northern winters.
In addition to affecting northern winters it has been known for a while that the Sun influences lightning.
For almost a century scientists have speculated that solar activity influences the rate of lightning strikes on Earth. Most studies have looked for a link between thunderstorms and the 11-year solar cycle. But recently it has been shown that polarity changes in the Sun’s magnetic field affects lightning rates.
The Sun’s magnetic field is drawn away from the Sun by the charged particles of the solar wind. It changes polarity along a twisted structure called the heliospheric current sheet (HCS), which spirals away from the Sun sometimes intersecting the Earth. When the HCS passes the Earth it was found that thunderstorms peaked within 1–2 days and the amount of lightning increased. It’s a robust result though the exact mechanism is not yet determined.
There is a very good case for the climate effects of high-speed solar wind streams and that the declining phase of the Solar cycle is especially significant for climate. The Solar Wind should be taken into account by the IPCC. In its last synthesis report it was not mentioned.