David.whitehouse@netzerowatch.com

Global surface temperature is, and always has been, the key climate parameter. Whatever is happening to the Earth’s climate balance, it must, sooner or later, be reflected in the global annual average temperature, and not just in regional variations. But therein lies what is to some an inconvenience as the changes in the global temperature this century is open to differing interpretations including the suggestion that increases in anthropogenic greenhouse gas emissions are not needed to explain the changes we have seen in the last 20 years or so.

It’s a conclusion that many would dismiss as coming from climate “skeptics,” or downright deniers. But what if it’s the view of scientists from two of the world’s leading institutes researching climate change; the University of Oxford and the US National Center for Atmospheric Research. Then it must be taken seriously and not dismissed offhand.

It is important research because it is the trend in the increase of global temperature caused by anthropogenic greenhouse gas emissions that is most important variable for policymakers considering the scale and timescale of action in the coming decades. However, this vital parameter is uncertain because recent decades have shown that were are living through a period of considerable natural climate variability.

Aerosol Emissions, the real culprit?

Thus, a new study published in the Journal of Climate suggests the effect of carbon dioxide this century might be small if not undetectable when compared to natural climate variability. The researchers contend that recent temperature trends might indicate that there is no detectable increase in global temperature due to greenhouse gas emissions.

While this suggestion is interesting it must be said that the researcher’s get themselves in a muddle when estimating temperature trends this century. On the one hand they acknowledge the existence of the global temperature hiatus between 2000 - 2014, but on the other hand they do not properly distinguish the effects of the natural El Nino events that have taken place in the past seven years. This is why they conclude there might have been an acceleration in global temperature increase over this period.

They say that most of increase is not due to greenhouse gasses but to aerosol emission reductions. The combustion of fossil fuels releases greenhouse gasses but it also causes pollution that cools the Earth, offsetting any warming. This is good news for public health as airborne particles kills several million people a year, but it also accelerates global warming.

Their assessment is that aerosol emissions have contributed to an increase in the rate of anthropogenic warming since 2000 although they have a large uncertainty. When considering estimates of the amount of warming due to aerosol reduction along with natural climate variability they find a solution with all the post-2000 temperature trends being due to natural variability alone. They say (p 4283) it’s a credible hypothesis that global temperature changes since 2000 could be “arising largely from internal variability.”

See how successful changes brought about to reduce emissions of particulates have improved health but also allowed more sunlight warmth (source EPA)