Solar variation to global warming

One alternative hypothesis to the consensus view that anthropogenic forcing has caused most of the recent temperature increase is that recent warming may be the result of variations in solar activity.

A paper by Peter Stott and colleagues suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[30] They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases. Another paper suggests that the Sun may have contributed about 45–50 percent of the increase in the average global surface temperature over the period 1900–2000, and about 25–35 percent between 1980 and 2000.[31]

Solar variation over the last thirty years.

A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming.[32] It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[33]

One predicted effect of an increase in solar activity would be a warming of most of the stratosphere, whereas an increase in greenhouse gases should produce cooling there.[34] The observed trend since at least 1960 has been a cooling of the lower stratosphere.[35] Reduction of stratospheric ozone also has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.[36] Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since.[1] In 2006, Peter Foukal and colleagues found no net increase of solar brightness over the last 1,000 years. Solar cycles led to a small increase of 0.07 percent in brightness over the last 30 years. This effect is too small to contribute significantly to global warming.[37][38] One paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays.[39] Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed this criticism of their hypothesis.[40] A 2007 paper found that in the last 20 years there has been no significant link between changes in cosmic rays coming to Earth and cloudiness and temperature.[41][42]

Feedback

Main article: Effects of global warming

When a warming trend results in effects that induce further warming, the process is referred to as a positive feedback; when the effects induce cooling, the process is referred to as a negative feedback. The primary positive feedback involves water vapor. The primary negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.[43] This provides a powerful negative feedback which stabilizes the climate system over time.

Water vapor feedback

One of the most pronounced positive feedback effects relates to the evaporation of water. If the atmosphere is warmed, the saturation vapour pressure increases, and the quantity of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO₂ alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer.[44]

Clouds

Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud, details that have been difficult to represent in climate models.[44]

Lapse rate

A subtler feedback process relates to changes in the lapse rate as the atmosphere warms. The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that with increased greenhouse gas content the rate of temperature decrease with height will be reduced, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.

Ice-albedo

Another important feedback process is ice-albedo feedback.[46] When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Rapid Arctic shrinkage is already occurring, with 2007 being the lowest ever recorded sea ice area. Some models suggest that tipping points exist, leading to a potentially rapid collapse of sea ice cover in the Arctic.[47]

Arctic methane release

Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the potentially rapid release of CO₂ and CH₄.[48][unreliable source?] Methane discharge from permafrost is presently under intensive study.[citation needed]

Clathrate gun hypothesis

Warmer deep ocean temperatures could also release the greenhouse gas methane from the 'frozen' state of the vast deep ocean deposits of methane clathrate, according to the Clathrate Gun Hypothesis, albeit over millenial time-scales. A further release of methane from shallow cold water clathrates is also expected, and is predicted to be faster.[49] Buffett and Archer predict a large release of methane in response to warming, and a large increase in methane stores if oxygen levels in the ocean fall.[50] They offer a "global estimate of 3×10¹⁸ g of carbon (3000 Gton C) in clathrate and 2×10¹⁸ g (2000 Gton C) in methane bubbles. The predicted methane inventory decreases by 85% in response to 3 °C of warming. Conversely, the methane inventory increases by a factor of 2 if the O2 concentration of the deep ocean decreases by 40 μM or carbon rain increases by 50%"

Sequestration

Ocean ecosystems' ability to sequester carbon are expected to decline as it warms. This is because the resulting low nutrient levels of the mesopelagic zone (about 200 to 1000 m depth) limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.