Introduction to man-made radiative forcing

We are at Mongstad - The largest oil refinery in Norway. Mongstad symbols an important driver of Norwegian economy - the oil. But it is also the largest emitter of CO2 in Norway, emitting the equivalent of 1/2 a million cars every year. But what are the main human influences on climate? What are the mechanisms that make them important? And can we make some simple physical reasoning to try to estimate the effect they have on climate? That is some of the questions I am going to try to answer in this lecture.

Proxy records of temperature indicate that that climate has been rather stable the last 2000 years, - until 150 years ago when a rapid temperature increase started. This rapid rise coincide with a period when world population have risen from just above 1 billion to 7 billions – and every day the population rises with another 200 000. In the same time economic wealth have increased by a factor of 10. As world population has grown and we have become richer, more energy is needed to sustain the world economy. We are today using more than 30 times the amount of energy we used 150 years ago. Currently almost 80% of that energy comes from the burning of fossil fuels.

We are now emitting over 1000 ton of CO2 every second. What is it that makes it so convenient for us to use fossil fuels? It is cheap, it’s plentiful, and it’s safe to use.

But maybe more important: it is energy dense. My energy source today is this sandwich. If this was made of oil instead of wheat it would be 5 times as energy rich. And this large energy density is what made oil the preferred energy source already a hundred years ago. It’s effect on climate was discovered much later. In 1957 the young scientist Charles David Keeling, began observing CO2 at the South Pole and the summit of Mauna Loa in Hawaii. And by 1961 it was established that atmospheric CO2 was steadily rising.

We now know from air bubbles in the Antarctic and Greenland ice sheets that the increase Keeling documented started at the same time as humans started its intense use of fossil fuels. This and several other scientific findings the last 60 years makes us sure that it is the use of fossil fuels that are the main reason for the large changes we have seen in the atmospheric content of CO2 the last 150 years. As more greenhouse gasses are emitted into the atmosphere more of the radiation that is trying to escape to space will be absorbed and re-emitted. Some will be emitted to space and some to the surface. This additional energy will make the surface warmer.

And it is this additional energy that we call the radiative forcing which is formally defined as the difference in radianting energy received by the Earth and energy radiated back to space before the surface temperature has responded to the change in radiation. If the difference is zero the earth is in radiative balance and we will have a stable climate. If the value is positive the earth will warm and if it is negative we will have a cooling.

The emissions of burning fossil fuels here at Mongstad do not only contain greenhouse gasses. It’s a source of aerosols, small particles containing soot and sulfate. Unlike CO2, which have an atmospheric lifetime of 50-100 years, these small aerosols only stay in the atmosphere for a few weeks. But despite their short lifetime they play an important role in human induced climate change. Aerosols is the second biggest effect that humans have on climate. Different aerosols scatter or absorb sunlight to a varying degree, depending on their physical properties. These scattering and absorbing properties are denoted as the “direct effect” of aerosols on the Earth’s radiation balance. As an example, pure sulfate aerosols reflect nearly all radiation.

An increase in these particles will cool the earth – in contrast, soot absorbs radiation, so increasing the amount of soot in the atmosphere will have a warming effect. Thus the climatic effects of aerosols are complex - some may cool the earth while others may induce a warming. To further complicate things they can also alter the climate by changing the optical properties of clouds. These effects are often called the aerosol’s “indirect effects”.

As mentioned previous a convenient index for measuring the importance of different factors on climate is the radiative forcing. However, this is a measure that cannot be directly observed, but since we know the changes in greenhouse gasses and aerosols fairly well, we can estimate the radiative forcing by using climate models. Using state of the art climate models shows that due to changes in greenhouse gasses the climate system now receives over 3 W/m2 more of energy than it did 150 years ago. This is partly counteracted by the cooling effect of the increased amount of aerosols giving a net change in energy of around 2.5 W/m2.

This increase in radiative forcing is a fundamental finding in climate science - it is this forcing that will drive the changes we’ve seen in temperature and other climatic variables. We now have an estimate of the main human induced changes in radiative forcing - so let’s go on and try to connect these changes to temperature changes. Our first attempt will be a simple model where we allow the surface temperature to respond to the additional energy provided by the radiative forcing, but we allow no other climatic parameters to change. If you are interested you can find the mathematical details of the model in the reading material. For simplicity we consider a doubling of CO2.

The response of the model is a temperature change of 1.2 ºC. Comparing this number with estimates from proper climate models we find that our estimation is a factor 2 or 3 too low. So why do we get this underestimation? - what effects is it that our simple model is not taking into account? - that is going to be the topic of next week’s lecture.