Forcing scenarios

There are certain things that drive the climate system, which one would not immediately think of, for example volcanoes. After Pinatubo erupted in 1991 in the Philippines, the ash it spewed out affected the climate, solar activity and, of course, the composition of the atmosphere for several years. We call such external factors forcing mechanisms, because, when they change, they force the climate to change.

Greenhouse Gases

Greenhouse gas concentrations in the atmosphere are well measured for the recent past, and so we do not, in this experiment, need to investigate any possible uncertainty. For the future, we are using 4 possible scenarios, the first two of which are taken from the 2001 Intergovernmental Panel on Climate Change report.

  • SRES scenario A1B
  • SRES scenario B1
  • Stabilisation at 400ppm – this assumes that legislation or alternative means cap emissions so that the concentration of CO2 in the atmosphere remains constant at 400ppm.
  • Sequestration at 2050 – this radical scenario assumes that in 2050 mechanisms are put in place to remove CO2 from the atmosphere.

Solar energy – past and future

For past solar energy (1920-2000), we have used 4 data sets of observations of the solar index (a measure of the amount of energy the Earth receives from the Sun). There is a reasonable amount of variation between these data sets, which are all based on observations. In case all of these substantially underestimate the actual trend in solar index, (in which case it could be argued that a large part of the observed warming in the second half of the 20th century might be caused by the Sun) we have arbitrarily created a 5th data set by doubling the trend in solar index in the Lean, Beer and Bradley data set.

In all the data sets, you can see the 11 year solar cycle, caused by a regular variation in sunspot activity on the Sun.

Why do the data sets all join at the start of the period, rather than at the end, when you would have thought observations were best? In fact, it would be have been better to do the latter, but, as we are starting all our simulations from a ‘spin up’ we have to be careful that there is not a sudden jump in a forcing at the join. In fact, it is the trend (or change over the period we are looking at) rather than the absolute value that really matters, so an offset at the start doesn’t matter. Reassuringly, all the data sets are at the same point in an 11 year solar cycle in 1920.

With regard to future solar energy: as no-one knows how the Sun’s energy output will vary over the next 80 years, we have created 3 scenarios – either the solar index will carry on increasing at the same rate it has increased over the past 80 years, or it will decrease at the same rate, or it will neither increase nor decrease. It is a reasonable assumption that reality will lie somewhere in between these cases.

Volcanic eruptions – past and future

Only volcanic eruptions large enough to force dust up into the relatively stable stratosphere have a significant effect on the world’s climate. Pinatubo, which erupted in 1992, cooled the Earth noticeably for about 2 years. Again, there is a reasonable amount of uncertainty in observations of volcanic emissions in the past – particularly in the pre-satellite era. For the past 80 years, we have created 5 data sets based on the Sato and Amman observations of volcanic aerosol in the stratosphere. This data is divided into 4 latitude bands of equal area – 90ºS-30ºS, 30ºS to the equator, the equator to 30ºN, 30ºN to 90ºN.

For the future, we have created 10 possible scenarios, as we have, of course, no idea what volcanoes may erupt where. One scenario simply repeats the recent past according to the Sato (2002) data set. Two more are based on observations of the preceding 80 years, based on the Sato and Ammann data sets. The remaining 7 are subsets of observations of 1400-1960, based on a data set constructed by Crowley.


Tropospheric and stratospheric ozone values are set according to observations, which are well constrained for the recent past. For the future, we use two scenarios – one is the IPCC B1 scenario, the other comes from the Hadley Centre, which predicts recovery of the ozone hole by about 2025.