The slower warming of global temperatures in the early 2000s has motivated many papers examining the modulation of global trends by climate variability. One question that has emerged is: what happens after a pause?
Observed global temperatures showed a 10-year trend of near-zero in the early 2000s, which is likely due to a combination of internal variability and natural forcings (solar & volcanic activity) temporarily offsetting the warming from greenhouse gases. When these temporary cooling influences are removed, what happens to global temperatures?
The figure (top row) shows the distribution of all 10, 15 & 20 year trends in a large ensemble (230 members) of simulations with the FAMOUS GCM with CO2 increasing at 1%/year. The mean trend is clearly positive, but there are also many periods of cooling, including several lasting 20+ years. The black curve shows the expected distribution using the control simulation, offset by the mean warming, highlighting that the control simulation alone provides a good estimate of the spread in possible trends.
If we then select all periods of the same length which occur immediately after periods which show a cooling then the distributions are shifted towards larger warming by around 20% (bottom row). In FAMOUS there is an increased likelihood of a faster than expected warming period (or ‘surge’) following a pause and this not just simply due to the removal of the cooling periods from the distributions.
So, will the real world similarly exhibit a ‘surge’?