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21st century warming trend change may not be due to greenhouse gasses, leading climate scientists say

Dr David Whitehouse, Science editor

A new study by a team of leading climate scientists suggests that the effect of carbon dioxide this century might be small when compared to natural climate variability.

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 various changes in the global annual average temperature, and not just in regional variations. But therein lies what is to some an inconvenience as the changes observed in the global temperature this century are open to differing interpretations including the suggestion that the influence of anthropogenic greenhouse gas emissions are not needed to explain what we have seen.

It’s a conclusion many would dismiss as coming from climate “sceptics,” 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 the 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 when compared to natural climate variability. The researchers contend that recent temperature trends might indicate that there is no significant increase 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 change in trend 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 most of the warming. This is good news for public health as airborne particles kills several million people a year, but it also accelerates global warming.

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

Corollary: “Sad and Misleading”

The scientists involved in this study have criticised this post in comments to the Australian Associated Press. Co-author Andrew Gettelman has said the article above was “totally misleading and is climate misinformation”. “Professor Myles Allen has said, “it would be funny if it wasn’t so sad.”

Such comments are both ridiculous and unfounded. The complainants should instead think about the process of scientific journalism and the very definition of the word ‘change.’ If you study their actual criticisms as opposed to their expressed annoyance it’s clear they have no case.

In his own words author Stuart Jenkins confirms what this post says quite unambiguously when he confirms to the AAP: 

“Essentially, we couldn’t rule out the possibility of the +0.1C/decade increase in warming trend being natural variability.” 

Now compare that to what I wrote about their paper: “It’s a credible hypothesis that global temperature trend changes since 2000 could be arising largely from internal variability.” It’s the same conclusion — although I added the word ‘largely’ so I am actually a little more cautious than Dr Jenkins. 

The criticism by Dr Gettelman is even more ridiculous when he says, “our study was discussing the recent past temperature record, and the statement only refers to the past 20 years.” Perhaps Dr Gettelman would like to look again at the title of this article.

The AAP fact-check is embarrassing. It is telling that the fact-checkers didn’t even bother to ask for any comment from the author of the very article they criticise. So let this stand as a riposte. 

But who checks the fact-checkers? They are splitting hairs but have no case. In fact, their own statements only confirm the gist of my review was accurate. 


Editor’s note: This post has been slightly revised.