In principle, all constituents of the atmosphere interact with the solar radiation traversing thru; thereby, they absorbs, reflects and scatters the solar radiation. Nevertheless, for most gases including GHGs, the reflected component is miniscule.

The question is how do GHG affect the reflected incoming solar radiation? Before addressing this question, let us examine the key components of the reflected solar radiation. They are 1) clouds; 2) Earth-surface albedo, and 3) aerosols. Among them, change in clouds holds the key to the final answer! The short answer is, the jury is still out on this. However, it is an important question, because small changes in the solar reflected back to space can make a big impact on the net global warming. Regardless, one thing is clear, the cooling by reflected solar can not entirely undo the GHG warming, otherwise there will be no warming and hence no change in clouds to counter the GHG influences. In other words, the influence of a cause can not totally annihilate its cause, because if did then the effect would have to persist without its cause. The convoluted argument is clear enough to convince those who believe GHG warming can be undone entirely by increasing clouds of the warmer atmosphere! The reality is most scientists remain unsure whether the feedback effect of changing clouds will be positive or negative while GHG induced global warming has become a fact of life!

The main greenhouse gases are CO2 and CH4. They do not reflect solar radiation, but just absorb it. So no counterbalancing effect can be created by them. What can create it are aerosols. Look for the abundant literature on the "dimming" and "brightening" of solar radiation, i.e. long-term negative or positive trends in surface solar radiation. The impact of anthropogenic aerosols (pollution) on these trends is quite clear.

Sulphate aerosols from volcanic eruption also have dimming effects. That is why there is some misguided push to inject artificial particulates in the high atmosphere to force a strong dimming effect, as a way to counterbalance global warming. However, this would also ruin the efforts of developing solar technologies as a way to lower carbon emissions--precisely the main cause of global warming.

So be careful about your conclusions!

coming back to the original question:

volcanoes emit ash particles but these are so big that they quickly fall back to the earth surface. Volcanoes emit the gas sulphur dioxide. This gas is transformed in the atmosphere to very small sulphuric acid droplets, as mentioned by dr Gueymard.

Such small droplets efficiently reflect solar radiation. Also the most important aerosol that causes the "dimming" is sulphuric acid. This comes from sulphur dioxide that is emitted in electricity power plants that use coal.

I agree with Harry.

More about geoengineering risks: Any increase in aerosol loading affects _direct_ irradiance about 3-4 times more than _global_ irradiance, which means that concentrating solar technologies (which are actively developed in many countries with high solar resource) would be very seriously affected, much more than flat-plate PV panels, for instance.

The use of geoengineering is a total paradox: Burning fuel to generate electricity releases GHGs that create global warming. Rather than addressing the direct cause of this problem, let's inject more particulates to decrease the incident solar radiation, and thus ruin the effort to cleanly produce electricity from the sun...

Ken

What about Clive Oppenheimer on Tambora and the year without a summer?

How much one wishes that  consequences of aerosol cooling and GHG warming were were easy-to-simulate to enable us to geoengineer our climate. The reality is that aerosols have large horizontal and vertical gradients causing corresponding influences on horizontal heating structures. Together with the aerosol effects on the optical properties of clouds, the joint outcome of aerosol and GHG effects will be difficult to assess reliably, but several works in this direction are in progress. Indeed, in such-like geo-engineering exercises, there is a fear of large unintended local consequences.    

Greenhouse gases cannot reflect sunlight, but clouds could do it if in a warmer climate the general circulation of the atmosphere increase cloud cover, cloud lifetime and or cloud albedo averages in global scale. However at the present moment nobody can affirm it is true. It is just one hypothesis that has to be tested yet. 

As a proof this topic is timely, there is a new paper just published today: http://www.atmos-chem-phys.net/14/7769/2014/acp-14-7769-2014.html

The geoengineering technique is referred to the sweet term of "solar radiation management". This very terminology sounds like an attempt to water down the issue by giving it an "inoffensive" term. Isn't it reassuring that some people just _know_ how to manage our atmosphere and climate? ;-)

Anyway, the paper also discusses the impact of this forced dimming on cloudiness and precipitation, and admits there are still many unknowns.

Kenneth, What you say is important indeed!

In their conclusion, they say "In this study, we have not considered the consequences of detailed stratospheric dynamics and sulfate aerosol chemistry on the ozone layer (Tilmes et al., 2009). Thus our model does not account for the ozone loss that might take place due to the increased stratospheric sulfate aerosols."

So at least they acknowledge the issue. Still, I would suggest you write a Letter to the Editor to explain the unintended consequences.

 In principle, all constituents of the atmosphere interact with the solar radiation traversing thru; thereby, they absorbs, reflects and scatters the solar radiation. Nevertheless, for most gases including GHGs, the reflected component is miniscule.

The question is how do GHG affect the reflected incoming solar radiation? Before addressing this question, let us examine the key components of the reflected solar radiation. They are 1) clouds; 2) Earth-surface albedo, and 3) aerosols. Among them, change in clouds holds the key to the final answer! The short answer is, the jury is still out on this. However, it is an important question, because small changes in the solar reflected back to space can make a big impact on the net global warming. Regardless, one thing is clear, the cooling by reflected solar can not entirely undo the GHG warming, otherwise there will be no warming and hence no change in clouds to counter the GHG influences. In other words, the influence of a cause can not totally annihilate its cause, because if did then the effect would have to persist without its cause. The convoluted argument is clear enough to convince those who believe GHG warming can be undone entirely by increasing clouds of the warmer atmosphere! The reality is most scientists remain unsure whether the feedback effect of changing clouds will be positive or negative while GHG induced global warming has become a fact of life!

Hi Joerg: good question; both NASA & NOAA have spent a lot of resources for monitoring Earth's radiation budget. Try to look at ERBE (old) CERES (new) data by Googling. Changes in Earth's reflectivity are among the key measurements. Best of luck.

-Yogesh.

Ken

As you very properly indicate there are natural variations from year to year. This is the reason that climate is defined as well as Climate Change. Climate is the average weather over a period sufficiently long that natural annual variation is averaged out. This means. A period of 30 years is used in meteorolgy for that reason meaning that climate change is the difference in average weather over one period of 30 years compared to that of a preceeding period of 30 years. Sonce we are now in 2015 it measn climate change is only defined for the period of 1985-2015 verssus that in the period 1955-1985 as the latest of the changes.

So what is the meaning of your single year 1998?

Ken

Well then good / bad for them. That is THEIR repsonsibility NOT supported by climatology

Ken

This here is RESEARCHgate. And: the question is about Climate Change not absolute temperatures and what does absolute temperature mean for an area, espcaially as large as a hemisphere