Climate Change and its Drivers
Despite its huge complexity Climate Change components can be broken down in to smaller constituent parts and more readily understood
The climate system is often considered to consist of five main sections:
- the atmosphere
- the hydrosphere – oceans, rivers and lakes
- the cryosphere – snow and ice cover
- the biosphere – the global sum of all living things
- the geosphere – rock, soil and the land surface
Trees and other plants use carbon dioxide to live and grow. Hence they are important in removing carbon dioxide from the atmosphere, that is, they form a carbon sink. When areas of forest are removed (`deforestation'), this carbon sink is removed and amounts of atmospheric carbon dioxide will remain higher than they otherwise would have been
Essentailly the Earth receives more energy from the sun at low latitudes so that’s near the equatorand less at the cold Poles. So the atmosphere is heated differentially and gets warmer at the Equator and colder at the Poles. Associated with that are then changes in pressure between the Equator and the Poles and that drives the atmosphere to circulate for the air to move in the atmosphere, that drives thewhole atmospheric circulation. The atmosphere essentially behaves very broadly speaking as a giant heat pump which is continuously trying to move heat away from the Equator and towards the Poles.
The other Climate forcings is volcanic activity because volcanic eruptions release large quantities of gases and particles into the atmosphere where they’re very good at backscattering the solar radiation that’s coming from the sun. So sunlight from the sun gets backscattered by the particles that volcanic eruptions put into the atmosphere. So generally speaking these lead to a cooling of the climate system although their effect is relatively short lived.
The most talked about way in which climate can be altered is the way that mankind is affecting the composition of the atmosphere by adding in particular the gas carbon dioxide to the atmosphere. Since we began to become industrialised in about 1750 we know that because we’ve been burning so many fossil fuels - that’s coal,oil and gas - to meet our energy needs for transport. So mankind has significantly increased the amount of carbon dioxide in the atmosphere since the onset of industrialisation and via the enhanced greenhouse effect
Water vapour is a strong greenhouse gas so as climate warms and we evaporate more water into water vapour we potentially warm the climate system even further because it’s a strong greenhouse gas. So there’s the, a sort of vicious circle threat what we call the positive feedback and warmer air can hold more water vapour than colder air as well. So there’s the potential for that to feed back on itself in a positive way. The other main thing to consider with the hydrological cycle is the changes to cloudiness along with this increased vigorousness of the hydrological cycle. We expect generally to see an increase in cloudiness with more water evaporated and recondensing to form clouds
Generally speaking high clouds are good at trapping long wave radiation that’s coming out from below so generally speaking high clouds tend to have a warming effect on the climate. On the other hand low clouds are better at reflecting the sun’s radiation back to space so they tend to have a cooling effect on climate
When we remove large areas of forest we remove what’s called a carbon sink because trees and plants use carbon dioxide from the atmosphere to live and grow. So they’re important in removing carbon dioxide from the atmosphere. Secondly when we clear the forest away we change what’s called the surface reflectance which as the name suggest is essentially just a measure of how reflective the surface is. Generally speaking dark-coloured surfaces such as evergreen forest absorb more solar radiation than light coloured surfaces such as desert or light coloured concrete. So if we remove areas of dark forest and replace them with lighter-coloured areas then we actually increase the reflectiveness of the surface and reflect more of the sun’s rays back to space.
And one other final influence of or effect of removing forest is that essentially we change what’s called the roughness length. That’s a measure of how the roughness of the very lowest level of the atmosphere lying on the surface that basically what that means is that basically affects turbulence and the way that weather systems form at very low levels
One of the ways in which that could be changed is that as climate alters and more rain falls in one place than another you could see a freshening of seawater which will ultimately lead to changes in the way the ocean circulates.
Changes in atmospheric temperature
- Melting of glaiciers that raise sea level
Changes in differential heating (latitudinal/longitudinal)
- Changes in tracks of weather patterns such as the Gulf Stream
Changes in solar output
- Increase snow melt reflects less heat back causing further warming
Changes in land use- e.g deforestation
- impacts carbon sink
- impacts solar radiation and reflection
- changes habitat
- affects roughness length - which affects turbulence and weather patterns
Changes in solar and volanic activity
- Are Climate forcings that effect climate withour being affected by it. ash for instance can backscatter radiation back into the atmosphere
Changes in soil moisture
- affects biosphere
Changes in human influences
- Increased ozone which is a pollutant in the troposphere
- Suplate and aerosols can cause back scattering which cool atmosphere
Changes in hydological cycle
Expected to become more vigorous as climate warms. Warmer waters causing more hurricanes and intensity of rainfall in certain areas
What drives the climate
climate is determined by the balance between incoming energy from the Sun and outgoing energy emitted by the Earth/atmosphere system. Figure 3 illustrates the situation in the annual and global mean
When we have greenhouse gases in the atmosphere the short wave radiation from the sun can still pass straight through, with the exception of ozone, which we’ll talk about later. But most of the greenhouse gases allow the short wave radiation from the sun to pass straight through. But they absorb strongly at the longer wavelengths typical of the surface and the atmosphere. So on the whole the greenhouse gases don’t absorb at the short wavelengths such as emitted by the sun. But the gas ozone does, it’s very good at absorbing in both the short wave and the long wave
We’ve got 342 Watts per metre squared of that radiation. The unit Watts per metre squared essentially means, 1 Watt per metre squared means that one Joule of energy is passing through every square metre of area every second. The digarm also show the long wave outgoing radiation
So those are the two energy outputs. To do the balance equation let’s add those together. 107 plus 235 exactly balances the 342 units that are coming in. So in this longer-term global mean we’ve got a balanced situation where as much energy is coming in as is going out
the energy is used at the surface to evaporate the
water, which is then held as vapour in the atmosphere. But when
that vapour condenses to form clouds, that energy is given back
and that’s called a latent heat flux
planetary albedo. If we consider the left region of the figure here,
we see that of the 342 units of radiation coming in from the sun,
107 units are reflected straight back to space so that they’re still
short wave radiation. That 107 units is made up of 77 units that are
reflected in the atmosphere by clouds and aerosol and also the 30
that are reflected by the surface. So the total 107 units reflected,
as a proportion of the 342 coming in in the first place, that’s roughly
a third and that’s what’s meant by the planetary albedo. It’s a
measure of the reflectiveness of the planet.
Of the 390 units of surface radiation, that’s the long wave radiation emitted by the surface, only 350 get absorbed by the atmosphere. As you can see the other 40 actually manage to escape directly into space. They do so because they can pass through what’s called the atmospheric window. And this is a region of the spectrum, so among all the different wavelengths there’s a region of the spectrum that contains wavelengths at which none of the gases in the atmosphere absorb at all. So it’s effectively literally just like an open window that allows radiation of those wavelengths to pass straight out to space. So if for instance we had a satellite instrument way above the earth looking down and measuring the radiation it sees coming from the earth and atmosphere, if it sees radiation in those particular wavelengths then we know that that radiation has originated at the surface. Okay and we call that region of the spectrum the atmospheric window.
The radiation balance at the top of the atmosphere is essentially a balance among three things
The incoming solar radiation (i) is balanced by the sum of the outgoing long wave radiation from the Earth/atmosphere system (ii) and the solar radiation that is reflected by clouds, aerosol, and the surface (iii).
Greenhouse Gases
The Earth is only capable of sustaining life because of the presence of so-called ‘greenhouse’ gases in its atmosphere. These gases – primarily carbon dioxide (CO2), water vapour (H2O), methane (CH4), nitrous oxide (N2O) and ozone (O3) – act as a blanket and keep the Earth approximately 33 °C warmer than it would otherwise be.
The figure below illustrates the case for a real greenhouse in very simple terms. The glass is transparent to the relatively short-wavelength incoming radiation from the Sun, which therefore passes through to the ground and warms it. The heat energy re-emitted by the surface, however, is of longer wavelength than that received from the Sun and the glass is not transparent to it – some therefore gets reflected by the glass and the interior of the greenhouse warms up as a result.
When human activities increase the amount of greenhouse gases in the atmosphere, initially the emission from the atmosphere increases, so the ‘back radiation’ from the atmosphere to the surface increases, tending to warm the surface. Almost every process except incoming solar radiation will potentially change after a period of adjustment. As examples, the increased surface temperature means the amount of surface radiation increases – also, since cloudiness has most probably changed, so too has the amount of solar radiation reflected by clouds. These are just two examples of many potential changes once the climate system has had time to adjust to the forcing
