A wise tutor once told me that if something is written down in a scientific textbook it is probably true, but if it is published in a journal then put it on probation. It was good advice, especially when it comes to climate science. Many is the time that a particular paper, or indeed a suite of them from the same stable, is latched onto by one side or another as proving a point or winning an argument, sometimes paraded in the media as such, for subsequent papers published perhaps sometime later, to show that the original findings were not that clear cut. Science has run its self-correcting course though often advocates don’t keep up and remain quoting the original paper as though it was gospel. The moral is that, especially in climate science, look hard enough at almost any research paper and you will raise more questions than answers. It’s inevitable.
It’s with this attitude that I approach a recent paper in Nature by Shakun et al.
It’s a fascinating study that overturns what was though to be the role of CO2 after the last glaciation. It claims that rising temperatures were preceded by rising CO2. This, of course, throws an interesting light on the rise in CO2 since the Industrial Revolution that the IPCC says is affecting out climate, dominating the warming, since 1960-80. The paper says, “Global temperature reconstructions and transient model simulations spanning the past century and millennium have been essential to the attribution of recent climate change, and a similar strategy would probably improve our understanding of glacial cycle dynamics.”
The curious thing is that if you look at the rise of CO2 in the atmosphere in the past 20,000 years or so and compare it to the rise in temperature, if you use data from Antarctic ice cores you find that the temperature leads the CO2 rise, as it seems to do in previous glaciations. But Shakun et al extend their analysis of global temperature to include 80 proxy stations scattered more evenly across the globe, though almost all of them very close to the oceans. The temperature proxies include ice cores, isotopic ratios, pollen, microfossils and a few others.
When those proxies are included the relationship between the CO2 increase and the temperature increase is reversed. This time the CO2 predates the temperature increase. The paper’s figure 2 (above) shows the results
The paper has attracted some comment centered around two issues. One is the method of area-weighting to produce a global mean temperature of the 80 proxy sites. The other has been about what the proxies themselves show and it has been suggested that, whilst they all show a rise in temperature since the last glaciation, they do not all have the same timings, and some are not useful at all. Producing an average temperature curve from this sample worries me.
The paper's figure 2 shows that the temperature increased by about 3.5 deg C between about 18,000 BC and 10,000 BC. During the same period CO2 increased by 70 ppm, from 190 to 260 ppm. Pre-industrial values for C02 are 280 ppm and today they are 390, a rise of 110 ppm. During this time the global temperature has increased by about 0.8 deg C. There are clearly other things going on.
Lean and DeLand in the Journal of the American Meteorological Society is another fascinating paper relating to the question of the variability of the Sun’s spectrum. #It was suggested recently by researchers that variations in the Sun’s spectrum, in particular the out of step changes in optical and Ultra-Violet radiation may mean that during solar minimum, when total solar irradiation (TSI) is at its minimum, more energy might be deposited in the earths atmosphere because UV radiation was in antiphase with the TSI.
This remarkable conclusion was based on observations made by the Spectral Irradiance Monitor (SIM) on the Solar Radiation and Climate Experiment (SORCE) spacecraft. It suggested that between 2004 and 2008 the Sun’s visible and Infra-Red irradiance increased as the TSI decreased as the Sun headed towards minimum. At the same time UV decreased 3-10 time more than expected.
Clearly, as the Sun is the primary driver of the terrestrial weather system understanding how its radiant energy is deposited in the atmosphere and on land is of vital importance. It’s well known that over the solar cycle the TSI varies by about 0.1 % and recently it was discovered that over a solar cycle the UV radiation varies by a greater proportion. Only the SIM however scrutinized these changes simultaneously showing that as the sun ended cycle 23 it displayed a 0.3 W per sq m decrease in UV at the same time as a 0.3 W per sq m increase in visible energy.
This changed many looked at the Sun-Earth interaction suggesting it was the often-discussed amplifying factor whereby small changed in TSI (0.1 %) which were seemingly negligible in climatic terms could be influencing the Earth’s climate to a greater extent than that variation suggested. It would have mean a reappraisal of solar-terrestrial climatic interactions.
Lean and DeLand examine the calibration of the SIM during 2008 when the Sun was in a minimum level of activity and didn’t change much. They say they found trends in solar irradiance and solar activity that are unlikely to be solar in origin and that the effects previously noted are calibration effects are not real.
They say: “rather than revising current understanding of solar physics and solar variability effects on climate, what is needed is improved characterization of the SORCE SIM observations….To prevent future research following a path of unrealistic solar-terrestrial behavior, the SORCE SIM observations should be used with extreme caution in studies of climate and atmospheric change until additional validation and uncertainty estimates are available.”
I think that goes for the Shakun et al paper as well. My old tutor was right!