Climate sceptics often like to say that scientists ignore certain aspects. Is this true? Today we take one climate sceptic — an ex-NASA scientist — and rebuke the issues he claims get ignored by scientists. Then, we question his own scientific proclamations. Will they stand up under multiple climate scientists’ gazes?

Got a question about climate science? We’re continuing to run the Ask a climate scientist series on Rooted. Keep the questions coming by emailing me directly.

These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

Crikey reader Simon wrote in with a beauty:

These are actually not my questions — they are raised by notorious climate sceptic Roy Spencer who says the questions are never answered by scientists. So let’s answer them:

“The possibility that small changes in ocean circulation have caused clouds to let in more sunlight is just one of many alternative explanations which are being ignored.”

Has it been ignored? Does it make sense?

“Not only have natural, internal climate cycles been ignored as a potential explanation, some researchers have done their best to revise climate history to do away with events such as the Medieval Warm Period and Little Ice Age. This is how the ‘hockey stick’ controversy got started.”

Have climate scientists ignored natural climate cycles like the MWP and Little Ice Age?

“If you can get rid of all evidence for natural climate change in Earth’s history, you can make it look like no climate changes happened until humans (and cows) came on the scene.”

Is this true?

We’ll answer this question in a few parts. First up (I slightly reworded the question so that it could be answered correctly): Has small changes in ocean circulation caused clouds to let in more sunlight? Can this account for global warming and climate change? Has this theory been ignored?

Dr Chris Brierley, from the Dept. Geology & Geophysics at Yale University, responds:

Ocean circulation change and its connection to climate change is an area of active research, but is somewhat hampered by the sparsity of long observation records. It is clear that there have not been any large changes in the ocean circulation (which must you already know, as you are only asking about the small changes). The fourth IPCC report devotes nine pages to the topic of regional changes in ocean circulation and water masses. They start the executive summary of the section with “Key oceanic water masses are changing; however, there is no clear evidence for ocean circulation changes”.

Most research in the area is focused on the role of the Meridional Overturning Circulation (sometimes colloquially called the Ocean Conveyor Belt) in the North Atlantic. This has very likely varied in strength over the past 50 years, but “no coherent evidence for a trend in the strength of the meridional overturning circulation has been found” (IPCC, 2007). The shutdown of this circulation is thought to be responsible for the sharp return to icy conditions that occurred during the last deglaciation (called the Younger Dryas), however the pattern of climate changes from this time was substantially different from observations over the past hundred years.

Cloud feedbacks were noted by the IPCC as the key uncertainty in determining the response of the climate system to an increase in radiative forcing, such as carbon dioxide (Technical Summary). A cloud’s influence on the incoming sunlight is not the only climate consideration; one also has to worry about the influence of clouds on the transfer of infrared energy (heat). Many factors are involved in determining the radiative properties of a change in cloud cover, including the altitude of the cloud, its lifetime and the size of the water droplets inside it. Cloud cover is a closely monitored quantity, and its influence on sunlight (shortwave radiation) is also carefully watched by instruments such as NASA’s Clouds and the Earth’s Radiant Energy System (CERES) instruments and science team. There are a lot of scientists around the world looking at these data beyond just the science team. This data is such a high resolution it is possible to observe the cloud response to individual marine vessels (see, for example, the second image on this NASA webpage).

In summary, the topics of ocean circulation change and cloud feedbacks are being extensively studied, rather than ignored, by the scientific community. I have not read anything in the literature discussing the process you described, and I feel that were it strong enough to substantially contribute to climate change it would have been already discovered. It is perhaps worth reiterating that even if the process were important, it would most likely operate in response to anthropogenic forcing rather than independently from it. For the few properties with sufficient observations to perform such analyses, recent changes in the ocean have a large component that can be directly attributed to anthropogenic activity (Sect 9.5.1 of the IPCC’s fourth assessment report).

This answer was reviewed by Clare Murphy (Paton-Walsh) from the Centre for Atmospheric Chemistry at the University of Wollongong.

Second part of the question: “Not only have natural, internal climate cycles been ignored as a potential explanation, some researchers have done their best to revise climate history to do away with events such as the Medieval Warm Period and Little Ice Age. This is how the ‘hockey stick’ controversy got started.” Have climate scientists ignored natural climate cycles such as the MWP and Little Ice Age?

Zanna Chase, from the Institute for Marine and Antarctic Studies at the University of Tasmania responds:

I’m not aware of any scientists trying to ignore these natural climate events. On the contrary, they are extremely valuable to climate scientists as they test our ability to understand how the climate system works. They are studied intensely — 3206 papers on the Web of Science published in the past 10 years on the topic Little Ice Age Climate and 472 papers on the topic medieval warm period.

Third part of the question. “If you can get rid of all evidence for natural climate change in Earth’s history, you can make it look like no climate changes happened until humans (and cows) came on the scene.” Is this true?

Zanna Chase, from the Institute for Marine and Antarctic Studies at the University of Tasmania responds:

Earth exhibits natural climate variability, there is no question about that. There’s no reason that human-induced change cannot be occurring in addition to this natural variability, and this is what all of the available evidence suggests is happening. If models can reproduce natural variability by including all known natural forcing (this is broadly true but details are still being worked out), but cannot reproduce post-industrial variability without including anthropogenic forcing (this is definitely true), this is strong support for human-induced climate change.

These last two answers were both reviewed by E. Christa Farmer from Hofstra University.

Peter Fray

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