Atmosphere, Isotopes and the Polar Record of Global Climate
27 Oct 2006 - Interviews, Atmosphere & Space, Arctic, Antarctic
Dr Pieter Tans
© Dr Pieter Tans / Dr Pieter Tans
Dr. Pieter Tans is a Senior Scientist at the NOAA's Earth System Research Laboratory (ESLR). Dr. Tans is an acclaimed expert in carbon cycle and greenhouse gases. He has been studying atmospheric chemistry for over 30 years, utilizing every available resource from field measurements of gas exchange in current atmosphere to paleoreconstructions and numerical models of atmospheric transport. NOAA's ESRL Carbon Cycle Greenhouse Gases Group, headed by Dr. Tans, monitors several long-term observation stations, including one at Barrow, AK and one at the South Pole.
The Cooperative Institute for Research in Environmental Sciences (CIRES), based in Boulder, Colorado, USA interviewed Dr. Tans, on behalf of SciencePoles, about the current state of knowledge about greenhouse gases and his plans for future research.
There is a lot of controversy about the significance of CO2 emissions in the US. What would be your response to people who believe that anthropogenic emissions do not contribute to climate change?
The U.S. is responsible for about one quarter of the global emissions of carbon dioxide. The global annual emissions of CO2 from burning of fossil fuels are several times larger than the global net annual exchange of CO2 between the atmosphere and the oceans and between the atmosphere and plants. The recorded increase of CO2 in the atmosphere is the result. Similarly, the atmospheric increases of methane, nitrous oxide, and other greenhouse gases all attest to overwhelming human influence. The absorption of infrared radiation by all these gases is very well known, and is changing the heat balance of the earth. If you think that the higher concentrations of greenhouse gases will not have an effect on climate you need a reality check.
Why is it important to study isotopic ratios of atmospheric gases?
Isotopic ratios indicate different sources of the trace gases, or different destruction mechanisms. For example, methane produced by biomass burning has a higher ratio of 13C/12C than methane in average air, whereas methane produced in wetlands has a lower ratio. Destruction of CH4 by OH radicals is a little faster for 12CH4 than for 13CH4, but not by much. On the other hand, destruction by chlorine radicals is considerably faster for 12CH4 than for 13CH4. These different processes leave a different "imprint" on the isotopic ratio of atmospheric CH4.
The more we know about these differences in isotopic ratio in the atmosphere, the more we will understand the complex interactions between atmosphere and the rest of the climate system. The differences in isotopic ratio tend to be very small in the atmosphere, and thus very high accuracy is required for the measurements. For methane, the accuracy of the isotopic ratio measurement needs to be one part in 10,000 or better.
How well can we approximate atmospheric chemistry of past climates?
The polar regions are tremendous archives of past atmospheric states. The evidence that has been gathered from the air bubbles trapped in the ice for up to a million years has, with the aid of atmospheric chemistry's assessing the ratios of certain key gas isotopes, provided a store of knowledge about past climates.
Furthermore, we know many emissions from plants today, and presumably they were quite similar in the past. We also know from laboratory measurements the temperature-dependent reaction rates and absorption cross-sections ("probabilities") for certain wavelengths of light of many chemical species. Therefore, I think that photochemical modeling of past climates has significant credibility.
What about present day atmospheric chemistry? What kind of measurements do scientists take?
Dave Keeling started the CO2 measurements at Mauna Loa, Hawaii, in 1958, as part of the International Geophysical Year. At that time the atmospheric concentration of CO2 was not known, nor whether it was increasing, although several people, including Keeling, suspected that it could be increasing because of the burning of coal and petroleum. Our lab started in 1968 with measurements on Niwot Ridge, Colorado. Our program gradually grew into a global effort, with now about 60 fixed sampling sites, and several regular ship crossings in the Pacific and Atlantic.
In addition, regular vertical profiles of trace gases are measured by small aircraft, primarily over North America. The number of chemical species measured has expanded from initially only CO2, and include now about 40 different species, including the isotopic ratios of some. It is now a global effort, with labs from some two dozen countries participating. In general, the aim of these measurements is to spot trends in the growth rates and seasonal cycles, to find spatial patterns and changes thereof. Numerical models of global atmospheric mixing and transport are used to infer (changing) sources and sinks of the gases from the observed concentration patterns. We expect that the world will be trying to manage greenhouse levels in the atmosphere, and these measurements will provide essential feedback. This process has already started, in fact.
Can you speculate on the links between permafrost/sea ice/ice sheet melt and atmospheric chemistry?
The melting of permafrost, which is quite likely when the Arctic continues to warm, could have a major effect on the atmospheric CO2 concentration and climate. The amount of organic carbon that is now frozen in permafrost soils, and has therefore been preserved, is very large. It has been estimated to be between 500 and 1000 billion metric tons of carbon. For comparison, the total amount of carbon released as CO2 into the atmosphere by burning of coal, oil, and gas from 1850 until today is about 320 billion tons. When permafrost thaws, what will happen to that carbon? It is reasonable to expect that it will be turned into CH4 and CO2 gas by micro-organisms.
Do you plan any IPY-related activities? How would you assess the role of such international activities on the development of scientific thought?
We are collaborating with our Canadian colleagues to make new measurements in Northern Saskatchewan, and have plans to start, funding permitting, in northern Manitoba and Northwest Territories in order to keep track of the response of the carbon cycle to Arctic warming. Science is an international activity by its very nature.
By: Jean de Pomereu
