Hill Heat

Program Summary

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.

Biographies

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.

Earlier this week Alexandra Kougentakis, Fellows Assistant at the Center for American Progress, attended one of the American Meteorological Society’s seminars discussing the latest in climate science (which are great resources for policymakers, as they tend to take place on Capitol Hill). She has reported on the event for Climate Progress:

One of the favorite, though well-debunked, claims of the global warming skeptics is that the recent warming is due to the recent up tick in solar activity. The current solar cycle has indeed seen higher-than-average sunspots, but what most strengthens skeptics’ argument is the lack of knowledge about what this means. In that light, the American Meteorological Society’s recent seminar, – Solar Radiation, Cosmic Rays and Greenhouse Gases: What’s Driving Global Warming? – was especially illuminating

The core of the skeptics’ argument is to take legitimate scientific fact and distort it to serve a false premise. Solar activity is among the external factors listed by the IPCC whose variation could be a source of radiative forcing, which is the net change in solar ray penetration between the two atmospheric layers closest to the earth. In other words, solar activity is potentially a cause of climate change. The historic correlation between solar activity and climatic shifts seen in the paleoclimate record provides evidence to this effect. Since the 17th century, the record of 11-year cycles of solar irradiance, or brightness, charted through the analysis of tree rings and ice cores makes clear that solar irradiance has increased. A correlation has been observed between solar activity and climate shifts, at least up until the mid-20th century, when the connection became sharply weaker.

One of the strongest arguments against attempts to link solar activity to current warming has to do with inconsistencies in the solar signal.

As climate scientist Dr. Caspar Ammann pointed out at the AMS seminar, the atmosphere is divided into different layers between which the impacts of ozone of pollution vary. While the models to predict the layers’ temperatures are not yet perfect, it is understood that solar irradiance would warm both the troposphere, the layer closest to the earth, and the stratosphere, the next outer layer. The warming that the troposphere is experiencing now is accompanied by a cooling of the stratosphere, which signifies that the current flux in solar irradiance is not a complete explanation for current climate shifts.

Climate experts widely recognize that the sun is a minor player at best in the current warming trend, and this is due in part to the scope of its impact seen in the past. At the AMS seminar, solar physicist Dr. Judith Lean further made the point that it is not realistic to separate anthropogenic and solar effects of warming since they are occurring simultaneously.

There are a lot of different factors affecting climate, some natural, some anthropogenic, and likely some that haven’t been clearly identified yet. The interaction between these factors makes it difficult to establish a linear connection between temperatures and a single variable. Addressing the specific argument of the skeptics, however, the solar-climate connection alone cannot account for the level of warming that has occurred since the mid-20th century.

The major take-home message of the AMS seminar, highlighted by both Dr. Lean and Dr. Ammann, was that both the empirical record and modeling demonstrates that current warming trends are most directly explained by anthropogenic greenhouse gas emissions, more than any other factor. This will probably do little to silence the ranting of global warming skeptics, but it is important to understand that it is only too convenient to blame nature for the problems that we bring upon ourselves.