Not because the sun is glowing hotter but because the sun’s magnetic field intensified in recent years. The intensified magnetic field steered away much of the cosmin rays that would otherwise have penetrated earth’s atmosphere. And it turns out that cosmic rays are a key element for the formation of clouds. This according to Nigel Calder, former editor of New Scientist, writing in the TimesOnline that the role of greenhouse gases in global warming is overstated:

Henrik Svensmark in Copenhagen first pointed out a much more powerful mechanism.

He saw from compilations of weather satellite data that cloudiness varies according to how many atomic particles are coming in from exploded stars. More cosmic rays, more clouds. The sun’s magnetic field bats away many of the cosmic rays, and its intensification during the 20th century meant fewer cosmic rays, fewer clouds, and a warmer world. On the other hand the Little Ice Age was chilly because the lazy sun let in more cosmic rays, leaving the world cloudier and gloomier.

The only trouble with Svensmark’s idea — apart from its being politically incorrect — was that meteorologists denied that cosmic rays could be involved in cloud formation. After long delays in scraping together the funds for an experiment, Svensmark and his small team at the Danish National Space Center hit the jackpot in the summer of 2005.

In a box of air in the basement, they were able to show that electrons set free by cosmic rays coming through the ceiling stitched together droplets of sulphuric acid and water. These are the building blocks for cloud condensation. But journal after journal declined to publish their report; the discovery finally appeared in the Proceedings of the Royal Society late last year.

Thanks to having written The Manic Sun, a book about Svensmark’s initial discovery published in 1997, I have been privileged to be on the inside track for reporting his struggles and successes since then. The outcome is a second book, The Chilling Stars, co-authored by the two of us and published next week by Icon books. We are not exaggerating, we believe, when we subtitle it “A new theory of climate change”.

Fewer clouds means more sunlight reaches the earth’s surface, raising the globe’s average temperature. But, as Calder points out, there has been no overall change in global air temperatures since 1999. But it is not a new hypothesis, nor have Calder and Mr. Svensmark been the only scientists investigating the topic.

Consider for example, S. K. Solanki and N. A. Krivova, both of the Max Planck Institute for Aeronomy in Germany. They published an article, “Can solar variability explain global warming since 1970?” in the Journal of Geophysical research in 2003. The abstract states,

The magnitude of the Sun’s influence on climate has been a subject of intense debate. …

… the Sun cannot have contributed more than 30% to the steep temperature increase that has taken place since then [1970], irrespective of … cloud coverage affected by the cosmic ray flux.

Well, 30 percent actually seems quite a lot to me. The region of space affected by the sun’s magnetic field, btw, is called the heliosphere and its extent defines the limits of the solar system; beyond the heliosphere is interstellar space. The heliosphere is enormous, extending at least 50 astronomical units (AUs) beyond Pluto’s orbit. An AU is the mean distance of the earth from the sun, approximately 93 million miles.

Sunspots are surface indications on the sun of changes in the sun’s magnetic field. Spots wax and wane in an 11-year cycle (approximate). The Marshall Institute summarized in 2000 that since about 1980 have scientists learned that,

… the total light, or brightness, of the Sun also changes in step with the magnetic cycle. That means the Sun is a variable star brightening and fading in light every 11 years. The Sun brightens when the magnetism strengthens, e.g., at the peak of the 11-year sunspot cycle. …

… Observations of the Sun’s change in total light made by NASA satellites since 1978 show that the Sun brightens and fades in its total energy output in step with the 11-year magnetic cycle. The amplitude of brightness change over a cycle seems too small to cause significant global temperature change on Earth.[2] But hypothetical brightness changes of a few tenths percent sustained over decades, longer than the Sun has been observed by satellite, could drive terrestrial temperature to change significantly. That would be the simplest way for the Sun to cause global climate change.

The climate record indicates a solar influence of this kind. An example is the record of the Sun’s magnetism, which is a proxy for solar brightness change, whose direct measurements extend back only to 1978, and reconstructed land temperatures of the Northern Hemisphere over 250 years (Figure 4). The two curves are highly correlated over several centuries.[3] Those changes in the Sun’s magnetism are presumed to indicate changes in the Sun’s brightness.

Here a chart showing the amplitudes of sunspot activity, 1610-1990

Additional evidence points to the Sun’s signature in the climate record over millennia. Every few centuries the Sun’s magnetism weakens to low levels that sustain for several decades. An example is the magnetic low from ca. 1640 - 1720, called the Maunder Minimum, when sunspots were rarely seen (see Figure 2). The Maunder Minimum occurred within a several-centuries-long period of relatively cool temperatures and severe climate called the Little Ice Age.

In fact, “Tree growth records from Scandinavia and ice core records from Greenland covering the last 10,000 years show that 17 out of 19 coolings line up with lows in the Sun’s magnetism.”

Coronal holes:

Figure 3 – An image of the Sun taken in soft x-ray wavelengths on May 8, 1992 by the YOHKOH satellite. The bright features indicate gas at several million degrees K in the hot, rarefied gas of the outer layer of the Sun called the corona. Darker areas, especially near the poles, are called coronal holes. In them, the opened shape of the magnetic field allows the heated gas to stream into the solar system (image courtesy of YOHKOH Archive Center).

Now consider this:

Figure 5 – The percent of the Sun’s surface area covered by coronal holes (regions of open magnetic field carrying fast-moving charged particles from the Sun) and the globally-averaged satellite temperature record of the lower troposphere. The two curves are strongly correlated, except when internal changes of the Earth’s climate are present, e.g., strong El Niño events or volcanic eruptions, as labeled. The cause of the 11-year modulation in the temperature record may be either fast particles from the Sun or high-energy cosmic rays from the Galaxy or both.

The article is careful to point out that these are just correlations and no scientific conclusion of causation can be drawn from these data. In fact, a footnote cautions against assuming ‘that one knows the mechanism of solar change (i.e., total brightness), and the response of the climate to such change. Neither is known!” But this area of investigation is ripe for further inquiry, it seems, and no doubt will draw increasing attention. I commend the Marshall Center article to your attention.