Came across a nice paper today suggesting that Future climate forcing potentially without precedent in the last 420 million years, by Foster, Royer & Lunt (I say nice because I found it quite easy to understand, not because what it suggests is possible would be nice). The paper was mainly looking at why our climate (surface temperature) has been reasonably stable for a very long time (hundreds of millions of years).
The main factors that determine our climate are the amount of energy we get from the Sun (Total Solar Irradiance – TSI) and the greenhouse gas content of the atmosphere (mostly CO2). It is well known that a star like the Sun gets more luminous as it ages, with the TSI at time , , given by
where is the age of the Earth, and is the TSI today. The change in solar forcing with time, relative to today, can then be written as
where is the albedo (which is assumed to be constant). Since , is negative; i.e., the solar forcing is greater now than it was in the past.
However, despite the Sun actually getting more luminous, it’s thought that our climate was actually typically slightly warmer in the past than it is now; the ‘Faint Young Sun’ paradox. As this paper illustrates, the reason is probably because CO2 was higher in the past than it is now. The change in CO2 forcing is
where is the CO2 concentration at the time of interest and is the pre-industrial CO2 level.
Okay, this is getting a little long. What the paper then did is construct a CO2 time series for the last 400 million years. The equation above can then be used to determine the change in CO2 forcing over that time interval, while the second equation in this post can be used to compute the change in solar forcing over the same time interval.
As can be seen in the figure on the right (top panel), the CO2 forcing has – on average – dropped over the last 400 million years, while the Solar forcing has increased. When combined (bottom panel), since these are the two dominant factors the control our climate, you see that the net forcing has – on average – decreased slowly over the last 400 million years. This both explains why we’ve been warmer in the past, despite the Sun being fainter, and why our climate has been reasonably stable (reductions in CO2 roughly balancing the brightening of the Sun).
What the paper then did was to compare future changes in atmospheric CO2 and forcings with their estimates for past changes. As the top panel of the figure on the left shows, we have the potential to produce atmospheric CO2 concentrations that are comparable to, or potentially even greater than, the highest values in the last 400 million years. If we compare the combined CO2 and Solar forcing, the lower panel shows that even an emission pathway that might be quite likely (RCP6) could produce an increase in forcing that is comparable to the reduction that has occured over the last 400 million years. This would take us back to a climate similar to that during the Eocene. What is more, we have the potential to produce a change in forcing that could produce a change to our climate that is without geological precedent in the last half a billion years.
Anyway, I just found this quite an interesting paper, which gives a nice explanation for why our climate has been reasonably stable over geological timescales, why it was warmer in the past despite the “Faint Sun”, and also shows how we have the potential to make changes that could be unprecedented on geological timescales. It’s also interesting to consider why the change in CO2 forcing has almost exactly balanced the increase in Solar forcing. I guess that if this hadn’t been the case, we may not have been here to ponder this issue.