Thermal graphic with Earth's long term warming trend, showing temperature changes from 1880 to 2015 as a rolling 5-year average. Orange regions indicate temperatures warmer than the 1951-1980 baseline average. Blue represents temperatures cooler than the baseline.
We know that in recent years, as the effects of climate change have ratcheted up, climate scientists have attempted to determine whether extreme weather events (extended heat waves, droughts, 'thousand year' floods etc.) are more likely to occur, e.g. as CO2 concentrations have increased along with global mean temperatures. Associated with this line of research, specialists have also tried to ascertain the extent to which past extreme events can be attributed to human influences on global climate.
Now, a breakthrough appears to have been made as published in Geophysical Research Letters, April 2, 2016 by Andrew King et al. See abstract at link below:
While noting "numerous extreme events" for which effects may be attributed to human-induced climate change, the researchers also note:
" it is likely that for temperature extremes occurring over previous decades a fraction of their probability was attributable to anthropogenic influences."
The question at hand, of course, becomes what fraction of their probability? If ALL the extreme events could be tied to human influence then the probability is P = 1.0. If it is 1 out of 5 extreme events, then P = 0.20. (Based on the standard frequency model for computing an empirical probability). But technically we would expect the probabilities to vary somewhat with the category assessed, so expect to find differences in terms of heat waves, droughts etc. A more meaningful tag would therefore be "record breaking hot years". This category would not only subsume heat waves but also the median level of high night time temperatures for regions, which most researchers regard as a key global warming parameter.
In this case, King et al - based on examining past hot years and seasons that exceeded the range of natural variability (going back to 1900) - found that humans have likely caused the 16 most recent record- breaking hot years experienced up to 2014. This means that for this category and defined subset of data the probability for human -induced effects was 1.0.
They further found the human influence extended back in time to at least 1937. Their focus for the extreme heat events covered five regions, including: central England, central Europe, central U.S., east Asia and Australia. The question that arises is: Why has it taken so long to track back the human agency in these events?
The researchers, in fact, showed that the emergence of the human signal had been delayed in the same regions owing to a cooling period in the 1970s that arose from presence of particulates and aerosols. After the Clean Air Act and similar bills were passed, those particulates disappeared and the cooling was no longer as evident. Aerosols remained and gave way to the global dimming phenomenon which also concealed about a third of global warming. But when aerosols were also controlled - after similar legislation, global dimming receded and warming assumed dominance. With that change, the unmistakable human contribution became more apparent.
One of the other profound conclusions to emerge from the work is that minus human-induced climate change the recent record hot years, including 2014 and 2015 would not have occurred.
These documented changes and signals should get the attention of the climate change deniers, especially those who dispute any role for humans in global warming. It is abundantly clear by now that humans - pumping out dozens of gigatons of carbon each year, are responsible for at least 60 percent of the 0.6W/ m2 by which the temperature of the planet is currently out of balance. (Every 2 ppm increase in CO2 concentration increases the radiative heating effect by 2 W/ m2.)