I am not going to go into details about impacts of climate change, for example where it warms most, where it doesn’t warm much or where a semi-arid climate changes to arid but instead I will just be looking at the big picture. If you want a much fuller description of global warming then this wikipedia article is a good place to start.
The main points are:
- Greenhouse gases like carbon dioxide are being emitted into the atmosphere as pollution from energy production, transport, industrial processes and construction.
- Earth’s energy balance: Greenhouse gases reduce the amount of heat from the surface of the earth that radiates out to space.
- The Earth’s climate has always changed as a result of natural factors. The energy balance change that results from our greenhouse gas emissions currently swamps all natural factors, both positive and negative, and is the reason for recent temperature increases.
- Natural variability can disguise the effects of climate change on a short time scale.
- Climate feedbacks.
Greenhouse gas emissions
The main greenhouse gases emitted by humans are: carbon dioxide, methane, nitrous oxide, and ozone and various halocarbons. Carbon dioxide is by far the most significant of these greenhouse gases. Water vapour is also a greenhouse gas but behaves as a feedback rather than a forcing. The amount of water vapour in the atmosphere is dependent on the temperature of the atmosphere. The warmer the atmosphere the more water it can hold. The reason water is a feedback rather than a forcing is because of the speed at which water vapour equilibrium within the atmosphere is restored. If you removed all water vapour from the atmosphere, then through evaporation the atmosphere would be at 85% of the equilibrium level within 10 days and by 30 days be at 100%. In comparison, the removal of carbon dioxide from the atmosphere is measured in the hundreds of years and methane in the tens of years. As a result, Water Vapour amplifies the effect of any other forcing which is why it is called a feedback.
As of the end of 2006 about 7 gigatons of carbon dioxide (GtC) are emitted into the atmosphere each year. About 4 GtC are taken in by plants and the oceans leaving a net increase of 3 gigatons of carbon dioxide in the atmosphere each year. This net increase shows up in the increase in the parts per million of carbon dioxide in the atmosphere.
It is projected that as temperatures rise, the ocean, plants and soil will not be able to absorb so much carbon dioxide and at some point may even start emitting carbon dioxide. In this scenario the atmosphere becomes the carbon dioxide sink of last resort.
Earth’s Energy Balance
For the Earth to remain at constant temperature it must remain in energy balance. This means that the radiation from the Sun that is absorbed by the Earth must equal the radiation emitted by the Earth. Much of the radiation energy emitted from the Sun is in the visible spectrum, whereas most of the radiation energy emitted by the Earth is infra-red. The description of radiation as infra-red, visible, ultra-violet is based on the energy of the radiation, which can also be defined by its wavelength or frequency. Each descriptive term defines an energy band, for example visible radiation refers to radiation that ranges from red light which is at the low energy end of visible radiation to blue light which is at the high end.
The atmosphere allows most of the radiation from the Sun to reach the surface of the Earth and this solar radiation is reflected back into space or warms the surface of the Earth. However, the infra-red radiation emitted by the Earth is absorbed by the greenhouse gases in the atmosphere. When infra-red radiation is absorbed by greenhouse gas molecules the energy is reemitted as infra-red radiation in any direction or is converted into kinetic energy, adding to the temperature of the atmosphere. If you were looking at the Earth from the moon with your hypothetical infra-red glasses on, the surface of the Earth would appear to be somewhere within the Earth’s atmosphere, the infra-red radiation that you see would not be that emitted from the surface of the Earth but that emitted by greenhouse gas molecules in the atmosphere.
The troposphere is the lowest level of the atmosphere and is approximately ten kilometres thick.
There are three pieces to the puzzle that need to be put together here, the pieces are:
1. The temperature within the troposphere declines by about 6.5 degrees centigrade for every 1000 m increase in elevation. It gets colder when you climb a mountain.
2. The more greenhouse gases in the troposphere, the higher the level (elevation) in the troposphere that emits the infra-red radiation that escapes to space rather than being absorbed again by another greenhouse gas molecule.
3. The amount of energy radiated is a function of temperature, a hotter object radiates more energy than a cooler one.
The Earth emits less infra-red radiation to space because increased concentrations of greenhouse gases in the troposphere increases the level (elevation) in the atmosphere that emits infra-red radiation that escapes to space. This higher level is cooler than the air below and thus emits less radiation than the warmer air below it. As a result less energy is radiated and this is what produces the energy imbalance.
Over time the surface of the Earth and the atmosphere above it will warm up. If we stop increasing the concentration of greenhouse gases in the atmosphere a new warmer equilibrium will be reached, where the surface temperature and the temperature of the troposphere is higher and the infra-red radiation escaping to space equals the amount of solar radiation absorbed by the Earth.
Climate change that results from human activities, predominantly the emission of greenhouse gases, is called Anthropogenic Global Warming (AGW). Some pollution cools the planet rather than warms, for example aerosols like Sulphate. The London “pea soup” fogs in the mid tweentieth century were caused by these emissions and they are also the reason for much of the air pollution in Chinese cities today. These aerosols remain in the atmosphere for only a short time because they are washed out by rain, often making the rain acidic. Greenhouse gases remain in the atmosphere over much longer time scales and as a result our emissions of greenhouse gases has accumulated in the atmosphere and the warming effect swamps the cooling effect of the aerosols.
Natural Climate Variations
The Earth’s climate has always varied. These climate variations have either been localized or global. For example the Medieval Warm Period was not global whereas Snowball Earth was a global climate event. Most of the dramatic changes between ice ages and interglacials were global. However, how the climate changed in specific regions was often not simple temperature changes, but changes in precipitation, both amount and timing. I am not going to go into any detail about how climate change impacts different locations, because there are still areas of contention as to how climate change plays out in terms of the detail in relation to which areas become drier or wetter or just how hot a particular location becomes. There is also some indication that as climate change progresses some locations might become first wetter and then drier or the opposite. Because of the complexity and lack of certainty of the local impacts of climate change I think trying to go into details can confuse and leave the impression that there is uncertainty about whether human emissions of greenhouse gases causes climate change or not. Let me be clear, climate change as a result of the emission of carbon dioxide and other greenhouse gases is happening. The Earth is now so far out of energy balance as a result of AGW that the rate of current and future warming is possibly unprecedented. Even if we stopped emitting greenhouse gases now the Earth would warm by at least another 0.5 degrees. All natural factors that have an impact on the Earth’s energy imbalance are overwhelmed in comparison to the impact due to AGW. The small reduction in incoming solar radiation is far too small to have any significant compensating effect.
Recent warming of the Arctic has been dramatic. Global warming since the mid nineteenth century has been about 0.7 degrees celsius, but what the global climate models have been predicting is that the warming in the Arctic would be significantly greater. In recent years the warming in the Arctic has been well above the global temperature rise of 0.7 degrees matching the predictions of most of the models. This is one area where there has been near universal agreement as to the localized effects of climate change predicted by computer simulations of the atmosphere.
Why doesn’t the Earth just get warmer each year due to the increasing concentrations of greenhouse gases in the atmosphere?
The biggest factor here is the ocean and volcanic eruptions, but there are other factors as well. Temperature measurements do not uniformly cover the whole of the Earth and though each temperature measurement can be accurate the extrapolation to the planet as a whole is less accurate. This discrepancy also shows itself in the difference in the temperature record between the land-based and satellite-based temperature measurements. Both show an upward trend. However, the satellite record gave 2005 as the warmest year on record whilst the ground-based temperature record found that 2005 was the second warmest after 1998. How do we know which is correct? We don’t, but does it really matter when the trend for all records is increasing temperatures.
When large ice shelves break off and drift into warmer waters, the ice gradually melts. The conversion of ice into water takes a lot of energy which sucks up heat from the surrounding area. For example the break up of an ice shelf the size of the Larsen B shelf in Antartica is not a yearly event and as such has a one off impact on the temperature record.
The energy balance between the oceans and the atmosphere has an impact on the surface temperature. The ocean takes a long time to overturn. That is, for water to move from the upper layers of the ocean to deep in the ocean depths and back again takes many years. It takes about 800 years for the volume of water the same size as that of all the ocean waters to overturn. But this only represents an average, in some areas overturning happens much faster than that, whilst other ocean current systems take thousands of years for water which was once at the surface to return to the surface once again.
When an El Niño occurs the normal upwelling of cold water in the Pacific Ocean off the coast of Peru is reduced or stops and the ocean surface temperature in the tropical Eastern Pacific is higher than normal. A strong El-Niño has a global climate impact.
The reason 1998 was such a warm year, was because a strong El Niño occured. Whichever temperature record you look at, 1998 is exceptional.
Volcanos have a significant cooling effect due to the emission of aerosols which reflect incoming solar radiation. The eruption of Mt Pinatubo in 1991 which ejected over 20 million tons of Suplhur dioxide, more than any other volcanic eruption since Krakatoa in 1883 resulted in a cooling of the surface and lower atmosphere during the following two years.
The short term temperature record reflects both the underlying temperature trend and specific events that dominate in a particular year. The error in the temperature record is less significant.
There are numerous feedbacks that will affect global warming. Some of these feedbacks are: less reflected sunlight due to less snow and ice cover, increased carbon dioxide emissions from soil, reduced carbon dioxide absorption by oceans, increased methane emissions as the frozen peat in the permafrost thaws and methane hydrates bubble to the surface of the ocean as the oceans warm. Our confidence in our understanding of these feedbacks is nothing like the same as we have for calculating the temperature increase due to increased concentrations of greenhouse gases. Determining which feedbacks are important and which aren’t is not easy. The scientific papers on these feedbacks are more recent and as a result there has been less opportunity for the same critical response within the scientific journals. That does not mean we should discount the effects of the feedbacks but that there should be a degree of reservation when considering their predicted responses. These reservations work both ways in that the feedbacks could be either much less or more significant than predicted.
The positive climate feedbacks make dangerous climate change far more likely. What I mean by dangerous climate change is sea level changes that threaten large cities around the world by 2100, significant changes in rainfall patterns that result in reclassification of agricultural and semi-arid land to being arid resulting in mass migration, warfare and civil strife (possibly already occurring in Dafur) and changes that have a significant effect on total agricultural production.