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Cloudy climate change:[br]How clouds affect Earth's temperature.

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Earth's average surface temperature[br]has warmed by .8 Celsius since 1750.

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When carbon dioxide concentrations[br]in the atmosphere have doubled,

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which is expected before the end[br]of the 21st century,

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researchers project global temperatures

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will have risen by [br]1.5 to 4.5 degrees Celsius.

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If the increase is near the low end,[br]1.5 Celsius,

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then we're already halfway there,[br]and we should be more able to adapt

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with some regions becoming drier[br]and less productive,

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but others becoming warmer,[br]wetter and more productive.

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On the other hand, a rise of 4.5 degrees[br]Celsius would be similar in magnitude

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to the warming that's occurred since[br]the last glacial maximum 22,000 years ago,

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when most of North America was under[br]an ice sheet two kilometers thick.

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So that would represent a [br]dramatic change of climate.

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So it's vitally important for scientists[br]to predict the change in temperature

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with as much precision as possible[br]so that society can plan for the future.

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The present range of uncertainty[br]is simply too large

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to be confident of how best[br]to respond to climate change.

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But this estimate of 1.5 to 4.5 Celsius[br]for a doubling of carbon dioxide

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hasn't changed in 35 years.

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Why haven't we been able [br]to narrow it down?

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The answer is that we don't yet understand[br]aerosols and clouds well enough.

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But a new experiment at CERN[br]is tackling the problem.

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In order to predict how[br]the temperature will change,

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scientists need to know something[br]called Earth's climate sensitivity,

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the temperature change in response[br]to a radiative forcing.

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A radiative forcing is [br]a temporary imbalance

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between the energy received from the Sun[br]and the energy radiated back out to space,

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like the imbalance caused by an[br]increase of greenhouse gases.

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To correct the imbalance,[br]Earth warms up or cools down.

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We can determine Earth's[br]climate sensitivity

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from the experiment that we've already

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performed in the industrial age [br]since 1750

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and then use this number to determine[br]how much more it will warm

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for various projected radiative forcings[br]in the 21st century.

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To do this, we need to know[br]two things:

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First, the global temperature rise[br]since 1750,

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and second, the radiative forcing[br]of the present day climate

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relative to the pre-industrial climate.

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For the radiative forcings,[br]we know that human activities

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have increased greenhouse gases[br]in the atmosphere,

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which have warmed the planet.

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But our activities have at the same time[br]increased the amount

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of aerosol particles in clouds,[br]which have cooled the planet.

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Pre-industrial greenhouse gas[br]concentrations are well measured

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from bubbles trapped in ice cores[br]obtained in Greenland and Antarctica.

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So the greenhouse gas forcings[br]are precisely known.

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But we have no way of directly measuring[br]how cloudy it was in 1750.

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And that's the main source of uncertainty[br]in Earth's climate sensitivity.

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To understand pre-industrial cloudiness,

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we must use computer models[br]that reliably simulate

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the processes responsible for [br]forming aerosols in clouds.

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Now to most people, aerosols are the thing[br]that make your hair stick,

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but that's only one type of aerosol.

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Atmospheric aerosols are tiny liquid[br]or solid particles suspended in the air.

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They are either primary,

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from dust, sea spray salt[br]or burning biomass,

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or secondary, formed by gas to[br]particle conversion in the atmosphere,

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also known as particle nucleation.

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Aerosols are everywhere in the atmosphere,

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and they can block out the sun[br]in polluted urban environments,

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or bathe distant mountains in a blue haze.

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More importantly, a cloud droplet cannot[br]form without an aerosol particle seed.

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So without aerosol particles,[br]there'd be no clouds,

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and without clouds,[br]there'd be no fresh water.

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The climate would be much hotter,[br]and there would be no life.

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So we owe our existence[br]to aerosol particles.

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However, despite their importance,

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how aerosol particles form[br]in the atmosphere

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and their effect on clouds [br]are poorly understood.

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Even the vapors responsible[br]for aerosol particle formation

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are not well established

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because they're present in only[br]minute amounts,

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near one molecule per million million[br]molecules of air.

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This lack of understanding[br]is the main reason

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for the large uncertainty [br]in climate sensitivity,

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and the corresponding wide range[br]of future climate projections.

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However, an experiment underway at CERN,[br]named, perhaps unsurprisingly, "Cloud"

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has managed to build a steel vessel[br]that's large enough

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and has a low enough contamination,[br]that aerosol formation can,

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for the first time, be measured under[br]tightly controlled atmospheric conditions

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in the laboratory.

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In its first five years of operation,[br]Cloud has identified the vapors

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responsible for aerosol particle[br]formation in the atmosphere,

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which include sulfuric acid,[br]ammonia, amines,

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and biogenic vapors from trees.

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Using an ionizing particle beam[br]from the CERN proton synchrotron,

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Cloud is also investigating[br]if galactic cosmic rays

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enhance the formation of [br]aerosols in clouds.

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This has been suggested as a possible[br]unaccounted natural climate forcing agent

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since the flux of cosmic rays raining[br]down on the atmosphere

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varies with solar activity.

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So Cloud is addressing two big questions:

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Firstly, how cloudy was the [br]pre-industrial climate?

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And, hence, how much have[br]clouds changed due to human activities?

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That knowledge will help sharpen[br]climate projections in the 21st century.

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And secondly, could the puzzling[br]observations of solar climate variability

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in the pre-industrial climate be explained[br]by an influence

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of galactic cosmic rays on clouds?

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Ambitious but realistic goals[br]when your head's in the clouds.