Last week we introduced the concept of radiative forcing. We calculated how changes in radiative forcing influence the temperature. If we double the amount of CO2 in the atmosphere, we get a temperature increase of 1.2ºC - if no other climatic parameters were changed. This was less than half of what we get with a proper climate model. We are now at Geilo, a ski resort in Norway. If the climate warms, that is going to reduce the skiing season and impact the local tourism. But a reduced snow season is also one of several important factors that will help us explain why our temperature calculation was so low.
So why is the Earth’s temperature change not just a straight forward calculation depending on the strength of the radiative forcing? The answer lies in the complexity of the climate system. Temperature is not the only thing on earth that will change if we impose a radiative forcing. As the temperature change, this will influence the amount of water in the atmosphere, the clouds, melting and accumulation of snow and ice and a series of other climatically important processes. These are processes that again will influence temperature. -This is what we call climatic feedbacks. They are key factors in understanding how sensitive the earth is to changes in the radiative forcing. Feedbacks can be positive or negative. Positive feedbacks enhance the initial change.
Negative feedbacks damp the change. How these positive and negative feedbacks balance is what determines the temperature response to a change in radiative forcing. Individual climate-feedbacks are not something that we can easily observe or measure. To estimate their magnitude we have to use sophisticated climate models estimating the full climate system including land, ocean and the atmosphere. However we can get a simplified idea on how feedbacks work by consider a simple system that transforms inputs to outputs. In our case the input is the radiative forcing, the output is the temperature change, and the system is the climate system. In a case of no feedbacks the input/output behavior is straightforward.
If we consider a doubling of CO2, giving a radiative forcing of 3.7 W/m2. The increased energy amount will lead to a warming. The climate system response is to radiate more energy back to space, and thereby dampen the warming. We have previously seen that the temperature response of such a simple system is approximately 1.2ºC However in a system with feedbacks, the changes in temperature will give rise to positive and negative feedbacks. These will feed back on the initial temperature response and change it. And the total temperature response is then much more complicated than in a system having no feedbacks. So what is the main feedback in the climate system? The answer is water vapor.
150 years ago a thermo dynamical relation that is of great importance to climate was discovered. The fact that the amount of water vapor that can be can be stored in the atmosphere increases exponentially with temperature. The warmer it gets, the more water vapor can be stored in the atmosphere. As water vapor is a powerful greenhouse gas, this increase in water vapor will further increase the temperature and we have a powerful positive climate feedback. We currently think that this positive feedback makes the earth about twice as sensitive to changes in radiative forcing as it would have been if this climate feedback did not exist.
However, water vapor is also the main contributor to a negative feedback that will damp the effect of increased water vapor stored in the atmosphere. This is the lapse rate feedback, and it
works in the following way: The greenhouse effect is not only depending on the amount of greenhouse gases, but also the temperature difference between the surface and the part of the atmosphere which radiate out to space. So changes in the surface-atmosphere temperature difference may induce a feedback. In the case of water vapor, increase in atmospheric water vapor has a larger effect on temperatures in the part of the atmosphere which radiate out to space than at the surface, leading to a more efficient loss of energy to space. This induces a negative feedback which partly counteracts the positive water vapor feedback.
Another important source for feedbacks is clouds. Clouds cover almost 70% of the Earth’s surface and they play a dominant role in the Earth’s energy balance. As they influence both incoming solar radiation and the energy emitted to space from Earth, changes in clouds give rise to several feedbacks. On one hand, clouds absorb and emit terrestrial radiation. They are generally colder than the surface they overlie. This makes them act very much like greenhouse gases, warming the surface. However, clouds also efficiently reflect solar radiation and reduce the amount of solar energy reaching the surface. And this tend to cool the surface.
Because of the competing effects clouds have on solar and terrestrial radiation, the effect of changes in the radiative forcing on clouds is complicated. While still rather uncertain, state of the art climate models indicate that the net cloud feedback is a positive feedback.
During the northern hemisphere winter 15% of the Earth is covered in snow or ice. The high reflectivity of snow is cooling the earth. If the climate warms, the extent of snow and ice is going to be reduced. That will induce a positive feedback by reducing the Earth’s reflectivity. This is called the albedo feedback. In the current climate much of the snow and ice is situated quite close to the close to the poles where the solar radiation is relatively weak. As a consequence of this the albedo feedback is thought to be somewhat smaller than the previously mentioned feedbacks. For timescales of a hundred years - the water vapor, lapse rate, cloud and albedo feedbacks are the main radiative feedbacks.
They will determine how strongly the temperature will respond to a change in radiative forcing. We can now go back to our simple calculation of the temperature response to a doubling of CO2 and try to recalculate the temperature change using climate feedback results from our own state-of-the-art climate model. If we include the four mentioned feedbacks, the temperature response increases from 1.2ºC to 3.1ºC . Of these 1.9 degrees extra warming, the water vapor feedback and the associated negative lapse rate feedback accounts for around 1ºC. The clouds give us an extra 0.6ºC and the snow and ice changes another 0.3ºC.
In this lecture we have introduced the concept of climate feedbacks and we have gone through some of the most important feedbacks that operate on timescales up to a hundred years. We have seen that it’s the climate feedbacks that to a large degree govern the strength of the climate response to a given radiative forcing. But how quickly does the temperature respond to a radiative forcing? Does it take a year, a decade or a century? That is governed by another important climatic parameter that we are going to investigate in next week’s lecture.