What would Crikey readers ask a climate scientist? Quite a lot of questions, actually. Several months ago I wrote about a American Geophysical Union’s service where journalists can ask climate scientists questions about climate change science and receive clear, factual, peer-reviewed answers via email and in time for deadline. It was a program trialled during last year’s Copenhagen conference (with varying success), was supposedly re-launched in June, but now with a dedicated AGU Q&A manager (obviously more questions came in then originally anticipated…) the Climate Science Q&A Service was officially launched on Monday. It is still in pilot project stage though, which will run until mid-January.
It was initially reported by some media outlets (including The Los Angeles Times) that the program was specifically formed in order to mobilize scientists against climate sceptics — but the AGU vehemently denied this, and were quick to insist on a correction, stating that the AGU “aims simply to provide accurate scientific answers to questions from journalists about climate science…”:
“In contrast to what has been reported in the LA Times and elsewhere, there is no campaign by AGU against climate skeptics or congressional conservatives,” says Christine McEntee, Executive Director and CEO of the American Geophysical Union. “AGU will continue to provide accurate scientific information on Earth and space topics to inform the general public and to support sound public policy development.”
In an attempt to depoliticise the issue, the AGU are careful to point out that questions will only be answered about science, not policy.
And so begins Rooted’s “Ask a climate scientist” series. Over 20 questions were sent in by Crikey readers, which will all be answered over the coming weeks. If you have any others, just let us know. These answers are flying in to my inbox with two, three, four different scientists writing lengthy explanations and adding to each other’s answers in a most fascinating and fact-filled manner.
Crikey reader Paul asks:
While the weight of scientific evidence clearly points to anthropogenic causes for global warming, I can imagine that evidence from time to time comes up which points to the opposite. Example of theories like this, which I think have been fully debunked, are sunspot activity and the fact that the Antarctic is getting colder.
1. What are the most prominent examples of apparent and as yet unexplained non-anthropogenic causes for global warming?
2. What are the most prominent examples of apparent and as yet unexplained causes for global cooling? The purpose of asking questions like this is for climate change scientific community to play devil’s advocate with itself, rather than responding reflexively to denialist ‘evidence’.
Professor of Geology at Yale, Jeffrey Park responds:
This is a very good question, both parts of it. Every scientist might have his/her favorite examples, but I will give you mine. The least-well understood non-anthropogenic factor in recent climate change is solar variability. I used “climate change” rather than “global warming” for a reason. The aspect of solar variability that correlates significantly with the last century of climate data is the 11-year sunspot cycle. The correlation is detectable, but weak, less than 0.1 degree C in the global average of surface temperatures. However, temperatures at different places around the globe vary at roughly 11-year period with somewhat larger amplitude, maybe a third of a degree C, but these variations largely cancel in a global average. Past temperature fluctuations, both at particular locations and in the global average, are much larger for purely internal climate oscillations like ENSO (3-5 year period) and the interdecadal oscillation (15-18 years). The actual global warming over the last 100 years is roughly 1 degree C, bigger than these natural oscillations.
The mystery about the “decadal” temperature variation is how the solar variability translates into such a signal. The sun’s total heat output varies very little over an 11-year sunspot cycle, but its output of high-energy cosmic rays has a larger variation, several percent, over the cycle. How cosmic rays influence surface temperatures is poorly understood, but some long-term measurements within the stratosphere also correlate with the 11-year sunspot cycle, and the effect here seems larger. The data include the production of carbon-14 in the stratosphere, and the level of stratopheric ozone. Because most of the sun’s cosmic rays are absorbed high in the atmosphere, a larger effect up there makes sense. How it translates to a small, but detectable, signal in Earth’s surface temperatures is an unsolved problem.
Because the documented correlation of sunspots and surface temperatures in the last 100-years is quite small, solar variability is not a good candidate for causing recent global warming. However, we know that solar variability was larger in the late 17th century, when sunspots were rare and portions of the globe were significantly cooler than today. Other things were happening at the same time, e.g., volcanic emissions, that could also explain the cooler temps in Europe and North America in the late 17th century. So figuring out the solar mechanism might help explain past climate changes better.
The second part of your question regards “global cooling,” by which I take it you mean the interval between 1940 and 1975 when global-average temperatures flattened out and perhaps declined a bit. There are several hypotheses for this. The simple answer is that natural climate variability on multi-decadal time scales is large enough to overcome enhanced greenhouse warming for a few decades, but later will boost anthropogenic warming when the natural cycle reverses (1975 to now). That simple answer doesnt explain the mechanism, however. For this I defer to a paper from 2000 and 2005 by Levitus et al, titled Warming of the world ocean, 1955-2003.
These papers compiled subsurface ocean temperature measurements since 1950, and show that the subsurface ocean was warming during the mid-century interval that Earth’s surface temperature seemed to stagnate. This suggests that the “globe” was still warming while the surface that we live on seemed not to. This result is not too surprising. The ENSO (El Nino Southern Oscillation) climate signal involves a large quasi-cyclic heat exchange between the atmosphere and ocean. In the 1970s Earth’s global-average surface temperature oscillated by half a degree C or more, with the heat going into, then out of, the tropical ocean.
Hope this helps.
Dr. Thomas M. Cronin adds:
Almost every observed or paleoclimate (proxy-based) record of climate change, cooling and warming, over interannual to million-year timescales, has multiple hypotheses in the literature to explain it. Climate — including temperature — varies over all timescales. So let’s make some distinctions first.
The causes that we hear about most are what climatologists call externally forced climate changes due to changing greenhouse gas (GHG) concentrations, volcanic activity (especially large stratospheric events from low latitude volcanoes), aerosols (which partially explain mid 20th century cooling), and solar irradiance variability (which some call sunspot activity). These forcing agents affect earth’s energy balance and temperature. Often the climate response is strongly influenced (dampened, amplified) by other physical, chemical, or biological processes known as feedbacks. Some feedbacks are fast, like changes in atmospheric water vapor, some are slower, such as glaciers and ice sheet dynamics. Some are positive and amplify GHG induced warming, some area negative and dampen it.
However, externally forced changes to atmospheric energy balance are not the same as other processes that influence climate. For example, internal “unforced” climate variability occurs, in certain observed patterns called modes of variability. These include ENSO (El Niño), North Atlantic Oscillation, Pacific Decadal Oscillation and others. These occur over interannual to multi-decadal timescales, are not precisely regular or cyclic, and seem to operate without changes in radiative forcing (although forcing by GHGs could modify existing patterns). A large community is involved with the detection of climate change and its attribution to either external forcing agents mentioned above, or modes of internal variability.
Moreover, over longer timescale, many other processes affect climate. These include plate tectonics, changing solar output, orbital (also called ice age or Milankovitch) cycles, meteor impacts, major periods of volcanism and CO2 input into the atmosphere, mountain uplift and large-scale continental weathering (global biogeochemical changes), and even evolution (i.e., the appearance of land plants changing the global carbon cycle). All affect climate.
So in answer to your question, there are many, many types of observed climate changes over all timescales, most explained by hypotheses scattered throughout a large literature. Many, even those occurring over longer timescales and millions of years ago, are relevant to today’s discussions because atmospheric CO2 concentrations now far exceed any seen in over 800,000 years, and because previous periods of rapid change in the global carbon cycle were accompanied by extreme warmth, high sea level, reduced glacial ice, changes in ocean chemistry (“acidification”), and other patterns.