Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30
Lecture 7: The Global Temperature Change over the Past Century
For earlier times, we have several ways to infer the temperature, for example measurements of deuterium abundance and oxygen isotopes in ice, the spacing of rings in trees, and oxygen isotopes in corals.
The results of these measurements indicate that there has been a global warming of 0.6 degrees Celsius (or 1 degree Fahrenheit) during the 20th century, which is unprecedented over the last 1000 years. From the year 1000 to 1900, the global temperature remained approximately constant, with a small gradual cooling over the 900 years of only 0.1 to 0.2 degrees Celsius and shorter-term fluctuations of similar amplitude.
The Little Ice Age is often referred to as a natural change in climate that occurred in recent history. The Little Ice Age refers to a cooling of the climate that took place between 1450 and 1850 in Europe and the North Atlantic, where the climate had been warmer before 1450. This has been used to argue that the warming during the 20th century is not larger than climate changes of the past. The truth is that when the temperatures are averaged over the planet, the cooling during the Little Ice Age was not as large for all the Earth as in Europe and the North Atlantic. There is only a small cooling of less than 0.2 degrees that took place gradually over the last millennium, until 1900. This gradual cooling from 1000 to 1900 could be caused by the Milankovitch insolation variations and might perhaps be the slow cooling tendency towards the next ice age in the Earth.
The results are shown in this Figure from the Intergovernmental Panel for Climate Change .
Is the warming of the 20th Century caused by the greenhouse effect?
Earlier (in Lecture 3) we discussed the concept of radiative forcing . We know that the well-mixed greenhouse gases in the Earth's atmosphere (which are all the greenhouse gases except water vapor) cause a radiative forcing of 2.4 Watts per square meter at the present time. This radiative forcing is increasing with time as the concentration of these greenhouse gases increases. There is no doubt or uncertainty on the fact that these 2.4 Watts per square meter are at present warming the Earth.
The basic laws of physics (specifically, the law known as Stefan-Boltzmann law that specifies the radiation emitted by an object as a function of its temperature) tell us how much the Earth should warm under the influence of a radiative forcing, if there were no other radiative forcings acting , and if the Earth system would immediately warm up to a new equilibrium temperature in which there is an energy balance between incoming and outgoing radiation. This law tells us that the Earth surface should warm by 1 degree Celsius for every 3.3 Watts per square meter of radiative forcing, so this would predict a warming of 0.7 degrees Celsius relative to pre-industrial times. You might think this agrees very nicely with the observed warming; but this conclusion would be wrong, as we shall see now.
Reality is more complicated, however. First, the total radiative forcing is affected by other causes, some of them are feedback effects caused by the warming itself, and some are causes independent from the greenhouse gases and the warming. Second, the Earth takes a certain time to warm up under the radiative forcing and reach a new equilibrium with energy balance, so the warming is delayed compared to the time when the forcing is applied. In other words, after greenhouse gases increase, the implied warming is only realised some time later.
Feedback effects are positive when a change in the climate system induced by the warming due to increased greenhouse gases causes a further increase in the total radiative forcing relative to pre-industrial times, therefore enhancing the warming. Negative feedback effects are changes induced by warming that reduce the total radiative forcing, therefore decreasing the amount of warming.
Examples of positive feedback effects:
When the feedback effects are taken into account, the warming induced by an initial radiative forcing of 2.4 Watts per square meter is roughly doubled to 1.4 degrees Celsius. Clouds are the most important source of uncertainty in predicting the extra warming due to feedback effects. As a result of these uncertainties, different climate models predict somewhat different warming due to the greenhouse radiative forcing once these feedback effects are included: from 0.9 to 2.4 degrees Celsius.
This now shows that the predicted greenhouse warming is more than the observed one. The reason must be because either the warming is being delayed, or there are other factors that are producing a negative radiative forcing and cooling the planet, compensating for some of the warming due to greenhouse gases.
Other radiative forcings
There are other possible factors that can make the Earth's mean temperature vary: solar variability (if the amount of sunlight has increased, that can cause warming), volcanic activity (the dust emitted by volcanoes stays suspended in the atmosphere for several years and reflects sunlight, cooling the Earth), changes in land albedo due to deforestation (which tends to increase reflection of sunlight), sulfate aerosol particles emitted mostly by coal power plants, and other types of aerosol particles. Sulfate aerosols and other types of dust tend to reflect sunlight and cool the Earth, and in addition they modify the properties of clouds enhancing their reflection of sunlight. Another important particle is black carbon, or soot, emitted by coal power plants, diesel engines, and biomass burning, which results from incomplete combustion of the carbon in fuels. Black carbon particles absorb (and do not reflect) sunlight, warming the air in which they are suspended, adding a positive radiative forcing.
The radiative forcings from all these effects and their uncertainties are shown in this figure from the Intergovernmental Panel for Climate Change :
The largest uncertainty in these radiative forcings is that due to aerosol particles and their indirect effect on clouds. It may be that the sulfate aerosols that result from coal burning (which cause also acid rain) has been cooling the planet during the 20th century, masquerading in part the warming caused by increased greenhouse gases. These aerosol particles stay in the atmosphere for only about two weeks (they are typically swept down to the ground every time it rains), so they are not well mixed in the atmosphere and their cooling effect does not accumulate like that of greenhouse gases, but depends only on their present emissions.
The effect of black carbon is also rather uncertain. Recently it has been pointed out that black carbon may cause a larger positive forcing than estimated by the IPCC, in part because the black carbon particles continue to act after they are deposited on the ground if they fall over ice or snow, reducing sunlight reflection and helping melt the ice or snow faster.
The effect of the ocean: delayed warming
As the Earth warms up, the water in the ocean is the component of the climate system that takes the longest time to be warmed. It takes a lot of energy to warm all the ocean (in contrast, the atmosphere is much less massive than the ocean and it warms up relatively quickly as a radiative forcing is introduced). This implies that, as greenhouse gases increase, the incoming and outgoing radiation in the Earth is out of balance, because some of the incoming radiation is being used to warm up the ocean. Therefore, temperatures do not have to increase as much so that the outgoing radiation balances the incoming one again until the ocean warming has been completed. It is rather difficult to estimate the rate at which the ocean is taking up energy, but the best estimates indicate a rate of 0.3 Watts per square meter, not enough to eliminate a lot of the radiative forcing from greenhouse gases.
A misguided debate on the cause of the observed warming in the 20th century.
There is an ongoing debate in the media about the cause of the global warming in the 20th century which is misguided. One often questions if the warming is due to human emissions of greenhouse gases by arguing, for example, that the Sun may have increased in luminosity and caused the warming instead.
The problem with these arguments is that the reason we know that greenhouse warming is taking place is because we know how the concentration of well-mixed greenhouse gases has increased, and we know the radiative forcing they cause of 2.4 Watts per square meter, or 10000 Watts per acre, to 10% accuracy. These 10000 Watts per acre cannot just evaporate, surely they are there. If the Sun had increased in luminosity more than we think, that would only mean that the total positive radiative forcing by the Sun and the anthropogenic greenhouse effect would then be even larger, and in order to understand the observed warming of only 0.6 degrees, we would then need to assume that the poorly known negative radiative forcing from sulfate aerosols or other causes must be even larger to compensate for the increase in sunlight and the greenhouse gases.
Actually, the natural forcings on climate, solar variations and volcanic eruptions, are less uncertain than the anthropogenic effects from sulfate aerosols and black carbon. The reality is that we are lucky that the human emissions of sulfate aerosols have inadvertently caused cooling, and mitigated global warming during the 20th century.