Debbie Polson

Challenges in Quantifying Changes in the Global Water Cycle

AbstractUnderstanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellit...

Decreased monsoon precipitation in the Northern Hemisphere due to anthropogenic aerosols

The Northern Hemisphere monsoons are an integral component of Earths hydrological cycle and affect the lives of billions of people. Observed precipitation in the monsoon regions underwent substantial changes during the second half of the twentieth century, with drying from the 1950s to mid-1980s and increasing precipitation in recent decades. Modeling studies suggest that anthropogenic aerosols have been a key factor driving changes in tropical and monsoon precipitation. Here we apply detection and attribution methods to determine whether observed changes are driven by human influences using fingerprints of individual forcings (i.e., greenhouse gas, anthropogenic aerosol, and natural) derived from climate models. The results show that the observed changes can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. Anthropogenic aerosol, not greenhouse gas or natural forcing, has been the dominant influence on Northern Hemisphere monsoon precipitation over the second half of the twentieth century.

Detectable Anthropogenic Shift toward Heavy Precipitation over Eastern China

AbstractChanges in precipitation characteristics directly affect society through their impacts on drought and floods, hydro-dams, and urban drainage systems. Global warming increases the water holding capacity of the atmosphere and thus the risk of heavy precipitation. Here, daily precipitation records from over 700 Chinese stations from 1956 to 2005 are analyzed. The results show a significant shift from light to heavy precipitation over eastern China. An optimal fingerprinting analysis of simulations from 11 climate models driven by different combinations of historical anthropogenic (greenhouse gases, aerosols, land use, and ozone) and natural (volcanic and solar) forcings indicates that anthropogenic forcing on climate, including increases in greenhouse gases (GHGs), has had a detectable contribution to the observed shift toward heavy precipitation. Some evidence is found that anthropogenic aerosols (AAs) partially offset the effect of the GHG forcing, resulting in a weaker shift toward heavy precipita...

Strengthening contrast between precipitation in tropical wet and dry regions

The wet-gets-wetter, dry-gets-drier paradigm (WWDD) is widely used to summarize the expected response of the hydrological cycle to global warming. While some studies find that changes in observations and climate models support the WWDD paradigm, others find that it is more complicated at local scales and over land. This discrepancy is partly explained by differences in model climatologies and by movement of the wet and dry regions. Here we show that by tracking changes in wet and dry regions as they shift over the tropics and vary in models, mean precipitation changes follow the WWDD pattern in observations and models over land and ocean. However, this signal is reduced and disappears in model dry regions, when these factors are not accounted for. Accounting for seasonal and interannual shifts of the regions and climatological differences between models reduces uncertainty in predictions of future precipitation changes and makes these changes detectable earlier.

Estimating the Transient Climate Response from Observed Warming

AbstractThe transient climate response (TCR) quantifies the warming expected during a transient doubling of greenhouse gas concentrations in the atmosphere. Many previous studies quantifying the ob...

Detectable Impact of Local and Remote Anthropogenic Aerosols on the 20th Century Changes of West African and South Asian Monsoon Precipitation: D&A AEROSOLS AND MONSOON PRECIPITATION

Anthropogenic aerosols are a key driver of changes in summer monsoon precipitation in the Northern Hemisphere during the 20th century. Here we apply detection and attribution methods to investigate causes of change in the West African and South Asian monsoons separately and identify the aerosol source regions that are most important for explaining the observed changes during 1920–2005. Historical simulations with the GFDL-CM3 model are used to derive fingerprints of aerosol forcing from different regions. For West Africa, remote aerosol emissions from North America and Europe (NAEU) are essential in order to detect the anthropogenic signal in observed monsoon precipitation changes. The changes are significantly underestimated in the model, however. While natural (volcanic) forcing seems to also play a role, the dominant contribution is found to come from aerosol-induced changes in the interhemispheric temperature gradient and associated meridional shifts of the Intertropical Convergence Zone. For South Asia, in contrast, changes in observed monsoon precipitation cannot be explained without local emissions. Here the findings show a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975, and since then by the increase in local emissions offsetting the decrease of NAEU emissions. The results show that the aerosol forcing from individual emission regions is strong enough to be detected over internal variability. They also underscore the importance of the spatial pattern of global-aerosol emissions, which is likely to continue to change throughout the expected near-future decline in global emissions.

Process for Tracking the Evolving Perception of Risk During CO2 Storage Projects

The paper describes a process for constructing a risk register to be used to track the evolving perception of risk during a CO2 storage project. A project-specific risk register is developed through a structured elicitation process to determine initial perception of risk and through discussion with experts. Regular updating of the register by experts is used to track changes in the assessment of risk as the project progresses and inform decision and actions during the project with the aim of reducing risk to an acceptable level.