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An example of rather different opinions on mechanisms of how the Sun and the Solar Cycle may influence global warming.

Last modified October 09, 2009 08:49

An example of rather different opinions on mechanisms of how the Sun and the Solar Cycle may influence global warming. They could be complementary though. Do you have examples on this subject or other diverging views on agriculture, please send them to us. Kees Stigter, INSAM (cjstigter@usa.net)

Viewpoint One


Cosmic Ray Decreases Affect Atmospheric Aerosols And Clouds

 

TW-image1Researchers have traced the consequences of eruptions on the Sun that screen the Earth from some of the cosmic rays. (Credit: iStockphoto/Nicolas Delafraye)

 

 

 

 

ScienceDaily (Oct. 6, 2009) — Billions of tonnes of water droplets vanish from the atmosphere in events that reveal in detail how the Sun and the stars control our everyday clouds. Researchers of the National Space Institute in the Technical University of Denmark (DTU) have traced the consequences of eruptions on the Sun that screen the Earth from some of the cosmic rays -- the energetic particles raining down on our planet from exploded stars.

"The Sun makes fantastic natural experiments that allow us to test our ideas about its effects on the climate," says Prof. Henrik Svensmark, lead author of a report newly published in Geophysical Research Letters. When solar explosions interfere with the cosmic rays there is a temporary shortage of small aerosols, chemical specks in the air that normally grow until water vapour can condense on them, so seeding the liquid water droplets of low-level clouds. Because of the shortage, clouds over the ocean can lose as much as 7 per cent of their liquid water within seven or eight days of the cosmic-ray minimum.

"A link between the Sun, cosmic rays, aerosols, and liquid-water clouds appears to exist on a global scale," the report concludes. This research, to which Torsten Bondo and Jacob Svensmark contributed, validates 13 years of discoveries that point to a key role for cosmic rays in climate change. In particular, it connects observable variations in the world's cloudiness to laboratory experiments in Copenhagen showing how cosmic rays help to make the all-important aerosols.

Other investigators have reported difficulty in finding significant effects of the solar eruptions on clouds, and Henrik Svensmark understands their problem. "It's like trying to see tigers hidden in the jungle, because clouds change a lot from day to day whatever the cosmic rays are doing," he says. The first task for a successful hunt was to work out when "tigers" were most likely to show themselves, by identifying the most promising instances of sudden drops in the count of cosmic rays, called Forbush decreases. Previous research in Copenhagen predicted that the effects should be most notice-able in the lowest 3000 metres of the atmosphere. The team identified 26 Forbush decreases since 1987 that caused the biggest reductions in cosmic rays at low altitudes, and set about looking for the consequences.



Forgetting to sow the seeds

The first global impact of the shortage of cosmic rays is a subtle change in the colour of sunlight, as seen by ground stations of the aerosol robotic network AERONET. By analysing its records during and after the reductions in cosmic rays, the DTU team found that violet light from the Sun looked brighter than usual. A shortage of small aerosols, which normally scatter violet light as it passes through the air, was the most likely reason. The colour change was greatest about five days after the minimum counts of cosmic rays.

Why the delay? Henrik Svensmark and his team were not surprised by it, because the immediate action of cosmic rays, seen in laboratory experiments, creates micro-clusters of sulphuric acid and water molecules that are too small to affect the AERONET observations. Only when they have spent a few days growing in size should they begin to show up, or else be noticeable by their absence. The evidence from the aftermath of the Forbush decreases, as scrutinized by the Danish team, gives aerosol experts valuable information about the formation and fate of small aerosols in the Earth's atmosphere.

Although capable of affecting sunlight after five days, the growing aerosols would not yet be large enough to collect water droplets. The full impact on clouds only becomes evident two or three days later. It takes the form of a loss of low-altitude clouds, because of the earlier loss of small aerosols that would normally have grown into "cloud condensation nuclei" capable of seeding the clouds. "Then it's like noticing bare patches in a field, where a farmer forgot to sow the seeds," Svensmark explains. "Three independent sets of satellite observations all tell a similar story of clouds disappearing, about a week after the minimum of cosmic rays."



Huge effects on cloudiness

Averaging satellite data on the liquid-water content of clouds over the oceans, for the five strongest Forbush decreases from 2001 to 2005, the DTU team found a 7 per cent decrease, as mentioned earlier. That translates into 3 billion tonnes of liquid water vanishing from the sky. The water remains the-re in vapour form, but unlike cloud droplets it does not get in the way of sunlight trying to warm the ocean. After the same five Forbush decreases, satellites measuring the extent of liquid-water clouds revealed an average reduction of 4 per cent. Other satellites showed a similar 5 per cent reduction in clouds below 3200 metres over the ocean.

"The effect of the solar explosions on the Earth's cloudiness is huge," Henrik Svensmark comments. "A loss of clouds of 4 or 5 per cent may not sound very much, but it briefly increases the sunlight reaching the oceans by about 2 watt per square metre, and that's equivalent to all the global warming during the 20th Century."

The Forbush decreases are too short-lived to have a lasting effect on the climate, but they dramatize the mechanism that works more patiently during the 11-year solar cycle. When the Sun becomes more active, the decline in low-altitude cosmic radiation is greater than that seen in most Forbush events, and the loss of low cloud cover persists for long enough to warm the world. That explains, according to the DTU team, the alternations of warming and cooling seen in the lower atmosphere and in the oceans during solar cycles.

The director of the Danish National Space Institute, DTU, Eigil Friis-Christensen, was co-author with Svensmark of an early report on the effect of cosmic rays on cloud cover, back in 1996. Commenting on the latest paper he says, "The evidence has piled up, first for the link between cosmic rays and low-level clouds and then, by experiment and observation, for the mechanism involving aerosols. All these consistent scientific results illustrate that the current climate models used to predict future climate are lacking important parts of the physics".

 

Journal reference:

  1. Svensmark et al. Cosmic ray decreases affect atmospheric aerosols and clouds. Geophysical Research Letters, 2009; 36 (15): L15101 DOI: 10.1029/2009GL038429

Adapted from materials provided by Technical University of Denmark (DTU), via AlphaGalileo.

 

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Viewpoint Two

 

Cracking the Complex Links Between Solar Cycle and Climate Change

Friday, August 28, 2009

By William Pentland

The 11-year-solar cycle, the stratosphere and the tropical Pacific Ocean impact periodic weather patterns across much of the planet, according to new research released in the journal Science.

Weather patterns are closely correlated with minor variations in total solar energy reaching our planet during 11-year solar cycles.  The new research is the first to crack the complex interactions between those phenomena.

Researchers from the National Center for Atmospheric Research (NCAR) in Boulder, Colo. fed more than a century of weather data into three computer models to investigate how the slight change of 0.1 percent of solar energy reaching Earth affects global weather patterns.  The answer appears to be related to two particular areas impacted by the changing solar energy radiation associated with the solar cycle. 

 TW-image2

Chemicals in the stratosphere and sea surface temperatures in the Pacific Ocean respond during solar maximum in a way that amplifies the Sun’s influence on some aspects of air movement.  This can intensify winds and rainfall, change sea surface temperatures and cloud cover over certain tropical and subtropical regions, and ultimately influence global weather.

“The Sun, the stratosphere, and the oceans are connected in ways that can influence events such as winter rainfall in North America,” says NCAR scientist Gerald Meehl, the lead author of the paper. “Understanding the role of the solar cycle can provide added insight as scientists work over the next decade or two toward predicting regional weather patterns.”

The connection between peaks in solar energy and cooler water in the equatorial Pacific was first discovered by Harry Van Loon of NCAR and Colorado Research Associates, a co-author of the paper.  The contribution by Meehl and his colleagues is to document that two mechanisms that had been previously theorized in fact work together to amplify the response in the tropical Pacific. The team first confirmed a theory that the slight increase in solar energy during the peak production of sunspots is absorbed by stratospheric ozone.

The energy warms the air in the stratosphere over the tropics where the sunlight is most intense, while also stimulating the production of additional ozone there that absorbs even more solar energy.

Since the stratosphere warms unevenly, with the most pronounced warming occurring at lower latitudes, stratospheric winds are altered and, through a chain of interconnected processes, end up strengthening tropical storms and precipitation.

At the same time, the increased sunlight at solar maximum causes a slight warming of ocean surface waters, especially across the subtropical Pacific, where Sun-blocking clouds are normally scarce.

That small amount of extra heat leads to more evaporation, producing additional water vapor. In turn, the moisture is carried by trade winds to the normally rainy areas of the western tropical Pacific, fueling heavier rains and reinforcing the effects of the stratospheric mechanism.

The top-down influence of the stratosphere and the bottom-up influence of the ocean work together to intensify this loop and strengthen the trade winds.

As more sunshine hits drier areas, these changes reinforce each other, leading to less clouds in the subtropics, allowing even more sunlight to reach the surface, and producing a positive feedback loop that further intensifies the climate response.

These stratospheric and ocean responses during solar maximum keep the eastern Pacific even cooler and drier than usual, producing conditions similar to a La Niña event.

However, the cooling of about 1-2 degrees Fahrenheit is focused further east than in a typical La Niña, is only about half as strong, and is associated with different wind patterns in the stratosphere.

Earth’s response to the solar cycle continues over the year or two following peak sunspot activity. The La Niña-like pattern triggered by the solar maximum tends to evolve into a pattern similar to El Niño, as slow-moving currents replace the cool water over the eastern tropical Pacific with warmer water.

Again, the ocean response is only about half as strong as with El Niño, and the lagged warmth is not as consistent as the cold event-like pattern that occurs during peaks in the solar cycle.

Solar maximum could potentially enhance a true La Niña event or dampen a true El Niño event. The La Niña of 1988-89 occurred near the peak of solar maximum.

That La Niña became unusually strong and was associated with significant changes in weather patterns, such as an unusually mild and dry winter in the southwestern United States.

The Indian monsoon, Pacific precipitation and sea surface temperatures, and other regional climate patterns are largely driven by rising and sinking air in Earth’s tropics and subtropics.

The new study could help scientists use solar-cycle predictions to estimate how that circulation, and the regional climate patterns related to it, might vary over the next decade or two.

To tease out the elusive mechanisms that connect the Sun and Earth, the study team needed three computer models that provided overlapping views of the climate system.

One model, which analyzed the interactions between sea surface temperatures and lower atmosphere, produced a small cooling in the equatorial Pacific during solar maximum years.

The second model, which simulated the stratospheric ozone response mechanism, produced some increases of tropical precipitation but on a much smaller scale than the observed patterns.

The third model contained ocean-atmosphere interactions as well as the role of ozone. It showed, for the first time, that the two combined to produce a response in the tropical Pacific during peak solar years that was close to actual observations.

This entry was posted on Friday, August 28th, 2009 at 13:22 and is filed under Cleantech. You can follow any responses to this entry through the RSS 2.0 feed.

 

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