Particulates and soot
Ship tracks over the Atlantic Ocean on the east coast of the United States. The climatic impacts from particulate forcing could have a large effect on climate through the indirect effect.
Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, was observed from 1961 until at least 1990. The main cause of this dimming is particulates produced by volcanoes and human made pollutants, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion – CO2 and aerosols – have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO2 greenhouse gases such as methane. Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide.
In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the radiation budget. Sulfates act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, known as theTwomey effect This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect,Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.
Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[85] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[86] The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.[87]
Satellite observations of Total Solar Irradiance from 1979–2006.
Solar activity
Main articles: Solar variation and Solar wind
Solar variations causing changes in solar radiation energy reaching the Earth have been the cause of past climate changes. The effect of changes in solar forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effect,while others studies suggest a slight warming effect.
Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.Radiosonde (weather balloon) data show the stratosphere has cooled over the period since observations began (1958), though there is greater uncertainty in the early radiosonde record. Satellite observations, which have been available since 1979, also show cooling.
A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. Other research has found no relation between warming in recent decades and cosmic rays. The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.
The fact we still see a positive imbalance despite the prolonged solar minimum isn't a surprise given what we've learned about the climate system...But it's worth noting, because this provides unequivocal evidence that the sun is not the dominant driver of global warming.
In line with other details mentioned above, director of NASA's Goddard Institute for Space Studies James Hansen says that the sun is not nearly the biggest factor in global warming. Discussing the fact that low amounts of solar activity between 2005 and 2010 had hardly any effect on global warming. Hansen says it is more evidence that greenhouse gases are the largest culprit; that is, he supports the theory advanced by "nearly all climate scientists" including the IPCC.
Satellite observations of Total Solar Irradiance from 1979–2006.
Solar activity
Main articles: Solar variation and Solar wind
Solar variations causing changes in solar radiation energy reaching the Earth have been the cause of past climate changes. The effect of changes in solar forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effect,while others studies suggest a slight warming effect.
Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.Radiosonde (weather balloon) data show the stratosphere has cooled over the period since observations began (1958), though there is greater uncertainty in the early radiosonde record. Satellite observations, which have been available since 1979, also show cooling.
A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. Other research has found no relation between warming in recent decades and cosmic rays. The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.
The fact we still see a positive imbalance despite the prolonged solar minimum isn't a surprise given what we've learned about the climate system...But it's worth noting, because this provides unequivocal evidence that the sun is not the dominant driver of global warming.
In line with other details mentioned above, director of NASA's Goddard Institute for Space Studies James Hansen says that the sun is not nearly the biggest factor in global warming. Discussing the fact that low amounts of solar activity between 2005 and 2010 had hardly any effect on global warming. Hansen says it is more evidence that greenhouse gases are the largest culprit; that is, he supports the theory advanced by "nearly all climate scientists" including the IPCC.
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