Lessons from Past Climate Predictions: Wallace Broecker
As it turns out, Broecker has been fortunate, because the cooling effects of human aerosol emissions have roughly cancelled out the warming effects of human non-CO2 greenhouse gas emissions since 1975, and solar activity has been flat over that period. So the net effect of the factors which he did not take into account has been close to zero. However, Broecker was also smart; the dominant effect on temperature since 1975 has been from CO2, as he expected. It’s better to be lucky than good, but it’s best to be both.
Broecker anticipated the actual increase in CO2 very closely, predicting 373 ppm in 2000 and 403 ppm in 2010 (actual values were 369 and 390 ppm, respectively). Broecker also used an equilibrium climate sensitivity of 3°C for doubled CO2; however, his model’s transient climate sensitivity worked out to be 2.4°C for doubled CO2. Current climate models put equilibrium sensitivty at 1.5 times transient sensitivty, so Broecker effectively underestimated the thermal lag of the climate system, and the equilibrium sensitivity in his calculations was approximately equivalent to 3.6°C for doubled CO2 - a bit higher than today’s best estimates of 2°C transient sensitivity, 3°C equilibrium sensitivity.