Steven J. Vavrus

The Community Earth System Model: A Framework for Collaborative Research

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad com...

Can natural or anthropogenic explanations of late-Holocene CO2 and CH4 increases be falsified?

Concentrations of CO2 and CH4 in the atmosphere rose slowly during the millennia prior to the industrial era. Opposing explanations for these increases have invoked natural and anthropogenic sources. Here we revisit this argument using new evidence to see whether either explanation can be falsified (disproven, in the sense proposed by German philosopher Karl Popper). Two lines of evidence suggest that natural explanations for the CH4 increase are falsified: (1) the absence of any sustained methane increase early in seven interglaciations prior to the Holocene; and (2) weakening emissions during the last 5000 years from the two largest global sources of CH4 – north tropical and boreal wetlands. Consistent with this interpretation, a new synthesis of archeological data from southern Asia reported in this issue indicates an exponential increase in CH4 emissions from expanding rice irrigation during the last 5000 years. Neither the anthropogenic nor the natural explanations for the CO2 increase can at this point be falsified. Previous studies that rejected the early anthropogenic hypothesis based on the small size of early farming populations ignored a rich array of archeological and historical evidence showing that early farmers used much more land per capita than those in the centuries just before the industrial era. Previous interpretations of very small terrestrial (anthropogenic and other) carbon emissions during the last 7000 years based on the δ13CO2 record failed to incorporate credible estimates of very large carbon burial in boreal peat lands during the late Holocene. Allowance for larger burial in peat deposits requires much greater emissions of anthropogenic carbon to balance the δ13CO2 budget. The prevalence of downward CO2 trends during equivalent intervals early in previous interglaciations poses a major problem for natural explanations of the late-Holocene CO2 increase.

Late Holocene climate: Natural or anthropogenic?

For more than a decade, scientists have argued about the warmth of the current interglaciation. Was the warmth of the preindustrial late Holocene natural in origin, the result of orbital changes that had not yet driven the system into a new glacial state? Or was it in considerable degree the result of humans intervening in the climate system through greenhouse gas emissions from early agriculture? Here we summarize new evidence that moves this debate forward by testing both hypotheses. By comparing late Holocene responses to those that occurred during previous interglaciations (in section 2), we assess whether the late Holocene responses look different (and thus anthropogenic) or similar (and thus natural). This comparison reveals anomalous (anthropogenic) signals. In section 3, we review paleoecological and archaeological syntheses that provide ground truth evidence on early anthropogenic releases of greenhouse gases. The available data document large early anthropogenic emissions consistent with the anthropogenic ice core anomalies, but more information is needed to constrain their size. A final section compares natural and anthropogenic interpretations of the C-13 trend in ice core CO2.

Comparisons of atmosphere–ocean simulations of greenhouse gas-induced climate change for pre-industrial and hypothetical ‘no-anthropogenic’ radiative forcing, relative to present day

We compare climate simulations for Present-Day (PD), Pre-Industrial (PI) time, and a hypothetical (inferred) state termed No-Anthropogenic (NA) based upon the low greenhouse gas (GHG) levels of the late stages of previous interglacials that are comparable in time (orbital configuration) to the present interglacial. We use a fully coupled dynamical atmosphere–ocean model, the CCSM3. We find a consistent trend toward colder climate (lower surface temperature, more snow and sea-ice cover, lower ocean temperature, and modified ocean circulation) as the net change in GHG radiative forcing trends more negative from PD to PI to NA. The climatic response of these variables becomes larger relative to the changed GHG forcing for each step toward a colder climate state (PD to PI to NA). This amplification is significantly enhanced using the dynamical atmosphere–ocean model compared with our previous results with an atmosphere–slab ocean model, a result that conforms to earlier idealized GHG forcing experiments. However, in our case this amplification is not an idealized result, but instead helps frame important questions concerning aspects of Holocene climate change. This enhanced amplification effect leads to an increase in our estimate of the climate’s response to inferred early anthropogenic CO2 increases (NA to PI) relative to the response to industrial-era CO2 increases (PI to PD). Although observations of the climate for the hypothetical NA (inferred from observations of previous interglacials) and for PI have significant uncertainties, our new results using CCSM3 are in better agreement with these observations than our previous results from an atmospheric model coupled to a static slab ocean. The results support more strongly inferences by Ruddiman concerning indirect effects of ocean solubility/sea-ice/deep ocean ventilation feedbacks that may have contributed to a further increase in late-Holocene atmospheric CO2 beyond that caused by early anthropogenic emissions alone.

Recent spatiotemporal patterns in temperature extremes across conterminous United States

With a warming climate, understanding the physical dynamics of hot and cold extreme events has taken on increased importance for public health, infrastructure, ecosystems, food security, and other domains. Here we use a high-resolution spatial and temporal seamless gridded land surface forcing data set to provide an assessment of recent spatiotemporal patterns in temperature extremes over the conterminous United States (CONUS). We asked the following: (1) How are temperature extremes changing across the different regions of CONUS? (2) How do changes in extremes vary on seasonal, annual, and decadal scales? (3) How do changes in extremes relate to changes in mean conditions? And (4) do extremes relate to major modes of ocean-atmosphere variability? We derive a subset of the CLIMDEX extreme indices from the North American Land Data Assimilation phase 2 forcing data set. While there were warming trends in all indices, daytime temperature extremes warmed more than nighttime. Spring warming was the strongest and most extensive across CONUS, and summer experienced the strongest and most extensive decrease in cold extremes. Increase in winter warm extremes appeared weakening relative to the rapid 1950–1990 increase found in previous studies. The Northeast and Midwest experienced the most warming, while the Northwest and North Great Plains saw the least. We found changes in average temperatures were more associated with changes in cold extremes than warm extremes. Since 2006 there have been 5 years when more than 5% of the U.S. experienced at least 90 warm days, something not observed in the previous 25 years. The unusually warm first decade of 21st century could have been associated with the warm conditions of near El Nino–Southern Oscillation-neutral phase of the decade, and possibly amplified by anthropogenic forcing. The widespread, lengthy, and severe extreme hot events documented here during the past three decades underscore the need to implement thoughtful adaptation plans in the very near future, to the growing evidence of increasing warm extremes across United States.