Solar Radiation, Cosmic Rays and Greenhouse Gases: What's Driving Global Warming?
Monday, March 24, 2008
12:00 Noon - 2:00 pm
Russell Senate Office Building, Room 253
What are the relative contributions from the sun, cosmic rays, and greenhouse gases, to the observed warming in the late 20th century and what are their expected contributions during the 21st Century? How does this compare to natural climate variability of past centuries and millennia? What is the principle driver or drivers of global warming in the 20th and 21st centuries? How are cosmic rays different from solar irradiance? Are there direct measurements of solar irradiance changes over the last 30 years or so? If so, what do these measurements show? What are the signals of this solar variability in the Earth’s atmosphere, and how do climate models reproduce these? Are we likely to observe additional changes in solar irradiance in the future and what might such variability have as an effect on climate? How is the ozone layer affected by solar activity changes and how does it influence surface weather and climate?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. Judith Lean, Senior Scientist for Sun-Earth System Research, Space Science Division, Naval Research Laboratory, Washington, DC
Lean PowerPoint HTML Version
Dr. Caspar Ammann, Research Scientist, Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO
Ammann PowerPoint HTML Version
Separating Solar and Anthropogenic (Greenhouse Gas-Related) Climate Impacts
During the past three decades a suite of space-based instruments has monitored the Sun’s brightness as well as the Earth’s surface and atmospheric temperatures. These datasets enable the separation of climate’s responses to solar activity from other sources of climate variability (anthropogenic greenhouse gases, El Niño Southern Oscillation, volcanic aerosols). The empirical evidence indicates that the solar irradiance 11-year cycle increase of 0.1% produces a global surface temperature increase of about 0.1 K with larger increases at higher altitudes. Historical solar brightness changes are estimated by modeling the contemporary irradiance changes in terms of their solar magnetic sources (dark sunspots and bright faculae) in conjunction with simulated long-term evolution of solar magnetism. In this way, the solar irradiance increase since the seventeenth century Maunder Minimum is estimated to be slightly larger than the increase in recent solar activity cycles, and smaller than early estimates that were based on variations in Sun-like stars and cosmogenic isotopes. Ongoing studies are beginning to decipher the empirical Sun- climate connections as a combination of responses to direct solar heating of the surface and lower atmosphere, and indirect heating via solar UV irradiance impacts on the ozone layer and middle atmosphere, with subsequent communication to the surface and climate. The associated physical pathways appear to involve the modulation of existing dynamical and circulation atmosphere-ocean couplings, including the El Nino Southern Oscillation (El Nino/La Nina cycles) and the Quasi-Biennial Oscillation . Comparisons of the empirical results with model simulations suggest that models are presently deficient in accounting for these pathways.
The Sun's Role in Past, Current and Future Climate Change
Correlations of instrumental or reconstructed climate time series with indices of solar activity are often being used to suggest that the climate system is tightly coupled to the sun. Yet correlations have to be used with caution because they are not necessarily synonymous with cause-and-effect relationships. Therefore, it is critical to understand the physical mechanisms that are responsible for the signals. Independent tests can then be applied to validate or reject a hypothesized link. Spatial structures that are related to the processes that translate the solar influence into a climatic response can serve as such a test. A particularly powerful example is obtained by looking at the vertical extent of the solar signal in the atmosphere. While the 11-year solar cycle can be found and the signal is consistent with the expected physical response throughout the atmospheric column, the underlying trends in temperature, however, are inconsistent with increased solar activity. These differences in trends correspond to the response to an increase in atmospheric greenhouse gas concentrations.
Another way of evaluating the consistency of a sun-climate relationship can be gained from extending the time scale from the most recent solar cycles back over the instrumental period and further into the historical past. However, solar forcing is not the only factor affecting climate, and thus other influences should not be neglected. Examples of the danger of over-interpretation of a purported solar link arising from superposition of multiple forcings are the famous Maunder Minimum (a period in the second half of the 17th and the early 18th Century when hardly any sunspots appeared on the solar surface), and the early 20th century where a general but small increase in solar activity coincided with changes in greenhouse gas concentration. The sun probably played some role in both of these cases, but the occurrence or absence of volcanic eruptions and other influences might have been just as important.
Nevertheless, climate reconstructions suggest that a small, but persistent, climate response to solar variability exists on the global/hemispheric scale as well as in some regions. Solar forcing and volcanic activity appear to have driven the majority of global/hemispheric climate variations over the past Centuries. But from about the mid-20th Century onward, the sum of these natural factors is no longer consistent with the observed warming. Only anthropogenic forcings, such as greenhouse gas increases and emissions of aerosol particles, can explain the observed temperature record. This explanation is even stronger when the vertical structure of the trends is included in the explanation. Therefore, one can also predict that future natural solar variations will most likely be insufficient to counter the changes that we can anticipate from future increases in greenhouse gas concentrations.
Dr. Judith Lean is Senior Scientist for Sun-Earth System Research in the Space Science Division of the Naval Research Laboratory in Washington, DC. She has served on a variety of NASA, NSF, NOAA and NRC advisory committees, including as Chair of the National Research Council (NRC) Working Group on Solar Influences on Global Change and, most recently, the NRC Committee on a Strategy to Mitigate the Impact of Sensor De-scopes and De-manifests on the NPOESS and GOES-R Spacecraft. A member of the AGU, IAGA, AAS/SPD and AMS, she was inducted as a Fellow of the American Geophysical Union in 2002 and a member of US National Academy of Sciences in 2003. She is the recipient of a number of NASA research grants, in collaboration with other SSD and US scientists, and is currently a Co Investigator on the SORCE, TIMED/SEE, SDO/EVE and GLORY/TIM space missions. A US citizen since 1992, she has a Ph.D. in Atmospheric Physics, 1982, from the University of Adelaide, Australia and B.S. (with Honors) from the Australian National University (1975). The focus of her research is the Sun’s variability and its impact on the Earth system, including climate change and space weather. She has published over 100 peer-reviewed papers in journals and books, and delivered over 250 presentations documenting her research.
Dr. Caspar Ammann is a research scientist, in the Climate and Global Dynamics Division of the National Center for Atmospheric Research in Boulder, Colorado. He has a M.S. degree in Geography and Geology from the University of Bern, Switzerland and a Ph.D. in Geosciences from the University of Massachusetts. His primary research is focused on the climate of past centuries and millennia, and how the current changes compare to this natural background. He has reconstructed past climates as well as volcanic forcing from proxy (e.g., ice cores, corals etc..) records and then simulated climate variability and response to forcings in state-of-the-art coupled Atmosphere-Ocean-General Circulation Models. Currently, Dr. Ammann’s research awards include an National Science Foundation Collaborations in Math- and Geosciences multi-institution program award to develop new Bayesian Hierarchical Models to reconstruct climate from proxies with different resolution and uncertainties and a project to improve regional impact studies based on better representation of forced, natural climate variations. He is a collaborator in several efforts to understand the effects of natural forcings on past Arctic climate and to improve model representation of the external forcing from the sun and volcanoes. He is also the organizer of the IGBP-PAGES Paleoclimate Reconstruction (PR) Challenge to assess spatial reconstruction methods and a member of the NASA Living with a Star, Targeted Research & Technology Scientific Steering Committee. Dr. Ammann has authored or co-authored more than 30 peer-reviewed articles in scientific journals and books, and made over 200 scientific presentations to peer-scientific, professional and student, as well as public audiences.