PP52A-01
Carbon Dioxide and the Early Eocene Climate of Western North America
Concentrations of atmospheric greenhouse gases play an important role in determining the climate by way of emitting longwave radiation towards the Earth, thereby increasing the temperature of the surface and lower atmosphere above that which would be measured in the absence of these constituents. Carbon dioxide (CO2), a well-mixed greenhouse gas, is naturally added to the atmosphere through aerobic biological respiration and decay, volcanic eruptions, and dissociation from ocean water. Meanwhile, it is removed from the atmosphere through photosynthesis, chemical weathering, and diffusion into the oceans. All of these factors make it difficult to pinpoint the concentration of atmospheric CO2 in the distant past. Indeed, estimates of CO2 concentration during the early Eocene (~ 55 million years ago) vary widely, from 300 ppm to upwards of 2000 ppm. This study employs a regional climate model to examine the effects of different CO2 levels on temperature and precipitation under early Eocene conditions. The region of interest, western North America, contains fossil evidence from the early Eocene that suggests regional rainfall was substantial enough and temperatures were warm enough to support subtropical vegetation, whereas today the region is primarily characterized by desert and steppe.
PP52A-02
Anthropogenic-induced changes in the 21st Century summertime hydroclimatology of the Northeastern US
Over the last 3 years, the Northeast Climate Impacts Assessment (NECIA) project has investigated the physical and socio-economic impacts upon the Northeastern US that may result from increased greenhouse gas emissions over the next century. In this presentation, changes in the summertime land/atmosphere hydroclimatology over the Northeastern US are investigated using a state-of-the-art regional climate modeling system. Results show that for the "business-as-usual" IPCC A1fi emissions scenario summertime precipitation is forecasted to increase over the northern and southern portions of the domain but decrease across the central portions; in contrast, evaporation is expected to increase across the entire domain. The balance between these two results in a decrease in summertime soil moisture content across most of the domain as well as an increase in the soil-moisture depletion rate that occurs during summer. At the same time, in the atmosphere there is an increase in both the specific and saturation specific humidity; the balance between these two results in an increase in relative humidity during the summer season. This increase in relative humidity, combined with an increase in overall temperatures, increases the daily maximum heat index for the region by 3.8C; for urban coastal regions the increase is even larger - up to 4.25C - resulting in an average heat index for the 5-month season that exceeds 32.2C (90F) - the level of "extreme caution" in today's climate. Finally, heavy-rainfall events are expected to increase across almost the entire domain, both with respect to frequency as well as intensity. In addition, the number of days which reach the "extreme caution" heat-index level are expected to increase by approximately 350% and could be reached every two out of three days in certain regions.
PP52A-03 INVITED
Ice Core Evidence for Amplification of the Recent Warming at High Elevations in the Tropics and the Likely Regional Impacts
IPCC (2007) models predict an enhancement of warming at higher altitudes throughout the tropics where temperatures may warm twice as much as the globally-averaged increase of 3°C predicted for sea level by 2100 AD. Ice core data collected over the last thirty years from low-latitude, high-elevation glaciers, along with continuous monitoring of selected sites, document this amplification and suggest an imminent demise of many of these ice fields. A new, annually resolved climatic and environmental record from the Quelccaya ice cap (5670 m asl) in Peru extends back to 315 AD. A new record from the higher, colder and drier Coropuna ice field (6450 m asl), 350 km southwest of Quelccaya and only 70 km from the Pacific Ocean, provides a much longer, albeit lower resolution, ~16,000 year history. El Niño-Southern Oscillation variations are recorded at both sites and document millennial scale variability. A series of ice cores drilled across High Asia provides climatic and environmental histories that also document the amplification of air temperature at high elevations. Regional impacts of this warming may already be underway. Observations in 2006 on Naimona'nyi (6100 m asl, also known as Gurla Mandata), located near the headwaters of the Ganges and Indus Rivers, indicate that under current climate conditions this ice field is not gaining mass. Ice cores from the Dasuopu glacier (7200 m asl) in the central Himalaya provide a high-resolution record of fluctuations in the intensity of the South Asian Monsoon. Reductions in monsoon intensity are recorded by insoluble dust and chloride concentrations. The deeper, older sections of the Dasuopu record reveal numerous arid periods, but none were longer and more intense than the 1790 to 1796 A.D. drought. This event is also prominent in the soluble aerosol records from the Quelccaya and Coropuna cores on the eastern side of the Pacific Basin, suggesting decadal-scale teleconnections between these regions. The similarities and differences among these high-resolution paleoclimate records for the last 1500 years will be discussed. The δ18O data from the high accumulation ice core sites around the Pacific Basin are compared with Niño 4 sea surface temperatures (SSTs) since the 1890s and appear to capture decadal-scale variations. Thus, they may offer the potential to extend the central tropical Pacific SST record beyond the instrumental period.
PP52A-04
Drought in the Western US and Floods in China: Pacific Response to Global Warming
Regional response to increased warming due to greenhouse forcing is of vital interest and is only now beginning to be explored. . The Pacific Ocean plays an important role in modulating regional climate in many parts of the world at interannual to decadal time-scales; the response of the Pacific to global warming is thus important to any future climate predictions. Currently, there are contradictory modeling inferences on the response of the Pacific to greenhouse warming. One way to resolve these contradictions is to look for analogues to global warming scenarios in the paleoclimate record. Although climate response to solar variability is likely to be different in magnitude and detail compared to greenhouse forcing, it is a potential analog. Solar modulation of Holocene climate has now been documented in a number of regions, including the southwestern US (SW) and the monsoon region of China, based on precipitation records of precisely-dated speleothems. In the SW higher than normal solar activity was associated with drier than normal conditions, typical of La Niña and the negative phase of the PDO. In contrast, higher solar activity was associated with wetter than normal conditions in China. This inverse response to solar forcing by the two regions is attributed to changes in the Walker circulation and the Pacific Decadal and El Niño/Southern Oscillation systems of the Pacific Ocean. The inverse response to solar forcing by the two regions is consistent with modern negative correlation (r = -0.45) of precipitation patterns between the two regions. If solar forcing is indeed a low-level analogue to greenhouse forcing, the speleothem data suggest that warming from greater greenhouse emissions will lead to La Niña and the negative phase of the PDO-like conditions in the Pacific. These conditions would result in greater drought in the western USA and wetter conditions in the monsoon regions of Asia.
PP52A-05 INVITED
Tropical Signature of Global Warming: El Niño or a La Niña?
Understanding the response of tropical atmospheric and oceanic circulation to increasing greenhouse gases is a central question in climate change research, since changes in tropical climate conditions can have far-reaching effects. There is a long-standing debate in the climate community on how the tropical Pacific will respond to an increase of greenhouse gases: will the structure of changes in the ocean surface temperature more closely resemble an El Niño or a La Niña? This dispute extends beyond global warming and has been a topic of intense interest within the paleo-climate community: El Niño and La Niña-like responses have been invoked as dynamical frameworks for interpreting past climate changes which occurred on timescales of centuries to millions of years. In this talk some of the theoretical, modeling understanding, and observational evidence for long-term changes to the tropical Pacific climate system will be highlighted. The dynamical arguments for an El Niño-like and a La Niña-like response will be described, contrasted and illustrated using climate model integrations of different levels of complexity. In models with a simplified representation of atmospheric physics, feedbacks originating in the ocean drive the system to a La Niña-like state. In models that include atmospheric general circulation components, atmospheric energy and mass balance constraints result in a reduction of the strength of the atmospheric overturning circulation – which is manifest primarily in the zonally-asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component. In these models changes over the tropical Pacific Ocean roughly resemble El Niño-like conditions; however, the mechanisms controlling these change are fundamentally different from those of El Niño. Even though modeling studies can help reconcile aspects of the diverging theoretical and model-based understanding, a true reconciliation requires observational evidence. Instrumental observations of sea level pressure indicate that over the 20th Century the Pacific Walker circulation appears to have weakened; however, it will be shown that in the instrumental record the differing reconstructions of historical sea surface temperature (SST) are inadequate to distinguish between an increase or decrease in East-West SST gradient across the Pacific. We outline what we view as a possible way forward, with paleo-proxy observations, to reconcile these diverging views. The problem of the Pacific response to global warming represents an important intersection of theory, modeling, direct observations and paleoclimate reconstructions. Theory and modeling have begun to converge in that robust mechanisms appear in a consistent manner in models of different complexity. However, the test of how these mechanisms operate in reality is in the hands of the observationalists, and the consequences for our understanding of the climate not only in the tropical Pacific, but in all the regions affected by ENSO, are great.
PP52A-06 INVITED
Nothing Like the Sun?
Will the response to greenhouse warming be unlike the response to increased solar radiance? The models used for the IPCC projections by and large agree that the future state of the tropical Pacific will be more El Niño-like (i.e. greater equatorial Pacific warming in the east than in the west), yet a number of paleoproxy records appear to be telling us that the past response to enhanced solar heating was more La Niña-like (i.e. a colder eastern equatorial Pacific). The period of instrumental data is ambiguous, allowing different interpretations of recent trends. (e.g Cane et al 1997 Science vs. Vecchi et al 2006 Nature). There are a number of possible resolutions: (1) the models are not correct; (2) the standard interpretation of the proxy data is not correct; (3) the response to greenhouse gases is different. An overview considering all three possibilities will be presented.
PP52A-07
Stability of Pacific Decadal Oscillation over the last 270 years
With the near certitude of the imprint of greenhouse warming on the global temperature record, climate scientists have shifted focus to more regional scale tests for greenhouse forcing in other records. A global warming signal has already been found in trends of mid-high northern latitude sea level pressure and zonal precipitation. But so far, there is no consensus on the effects of "future" greenhouse forcing on Pacific Decadal Oscillation–the most significant interdecadal oscillation in the largest ocean basin. Herein we test for a trend in the PDO by combining coral and tree ring proxies back 270 years. The high correlation (r=0.74) between these two proxies over a 147 years validation interval (1733-1880) justifies development of a joint proxy yielding a 0.91 correlation with the PDO from 1880 to 1992. The joint proxy indicates that the present state of the PDO is nearly "neutral", with no significant trend over the entire record. The stability implies that significant negative feedbacks identified for this mode are sufficiently strong to neutralize (so far) the effects of global warming on behavior of this mode.
PP52A-08
The Importance of Dynamical Responses to Forcing in Understanding Climate Changes Over the Past Millennium
We describe analyses of the dynamical response of the climate to different agents of radiative forcing over the past millennium that are likely in many regions to overwhelm global mean responses. Considered in particular are the responses of the Arctic Oscillation/North Atlantic Oscillation, the El Nino/Southern Oscillation (ENSO) and the Asian summer Monsoon to natural solar and volcanic radiative forcing. Our findings indicate varying importance of natural volcanic and solar radiative forcing depending on the temporal and regional scales considered. Comparisons against available historical and proxy data are made with respect to several different climate variables (surface temperature, sea level pressure, and precipitation/drought).