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2009 Literature Review Archives - Detection and Attribution
Hidalgo, H.G., et al. 2009, Detection and attribution of streamflow timing changes to climate change in the western United States. J. Climate, Vol 22, issue 13, pp 3838-3855, doi: 10.1175/2009JCLI2470.1.
Shifts in the seasonal distribution of streamflow in the western United States can be formally attributed to the effects of anthropogenic climate forcing.
Previous studies have identified a suite of hydrological changes in the western United States in the last 50 years including earlier snowmelt and spring runoff, changes in the proportion of rain versus snow events and in the water content of snow. These changes were thought to be related to anthropogenically-induced temperature increases in the region but this hypothesis had not been evaluated rigorously. In this study, the authors apply formal detection and attribution methods to evaluate the shift in the central timing of flow (date by which 50% of the water-year flow has passed) for the major hydrological regions of the western U.S. GCM control runs (no forcings) are used to represent natural flow variability in the region and simulations run with various forcings (anthropogenic and natural solar-volcanic) are used to evaluate the causes of the flow changes. The GCM data were downscaled to the region and used as input to a hydrological model to simulate flow in major western U.S. rivers. The results demonstrate that the observed trend to earlier central flow is outside the distribution of natural variability and that a portion of this change can be attributed to climate changes induced by anthropogenic GHGs, aerosols, ozone and land use changes. This paper is one of the first formal attribution and detection studies to be performed at a regional scale for a hydrological variable. The authors conclude that the changes in flow regime are in line with expectations for the early stages of climate change.
Hoerling, M., Kumar, A., Eischeid, J. and Jha, B. 2008. What is causing the variability in global mean land temperature? GRL 35, L23712, doi:10.1029/2008GL035984,2008; Compo, G.P. and Sardeshmukh, P.D. 2009. Oceanic influences on recent continental warming. Climate Dynamics 32:333-342.
Investigators report in two separate papers that much of the variability in temperatures over global land areas can be attributed primarily to changes in surface temperatures of the oceans surrounding them, rather than directly to internal land system variability or direct external forcing. In contrast, external forcing is an important factor in ocean temperature variability. Accurate modeling of natural internal ocean variability and ocean response to natural and anthropogenic external forcings will be essential for accurate projections of future changes in land surface climates.
Hoerling et al., in a paper published in the journal Geophysical Research Letters, use climate models to show that about 76% of annual global mean land temperatures can be attributed to changes in ocean surface temperatures. In contrast, ocean temperatures change primarily in response to internal variability or external forcing (human and natural), with most of the recent changes due to the latter. They also note that any internal variability within the land surface processes would need to be four times that observed between 1921-1970 in order for global mean temperature to be cooler than the 1921-1970 mean, an event they consider very unlikely to occur. In another paper, published in Climate Dynamics, NOAA scientists Compo and Sardeshmukh explain how variations in ocean temperature affect land temperatures. Rising ocean temperatures cause humidity of the atmosphere to rise and alter the vertical motion and cloud fields of the atmosphere. This, in turn, changes the long and short wave radiative fluxes over land, and hence land temperatures. They express concern that, currently, most coupled climate models inadequately replicate observed ocean temperature variability, particularly at the decadal scale. This could result in an underestimate of the role of internal natural ocean variability in attributing past climate change to causes. However, it also suggests that improving model ability to accurately simulate ocean temperature response to external forcing (whether due to changes in greenhouse gas and aerosol concentrations or natural causes) is an important factor in improving projections for future land temperature changes.
Jevrejeva, S., A. Grinsted and J. C. Moore, 2009, Anthropogenic forcing dominates sea level rise since 1850, Geophysical Research Letters, vol. 36, L20706, doi:10.1029/2009GL040216.
More than 70% of the total 18 cm sea level rise during the 20th century is attributed to increases in greenhouse gases.
A team of scientists has reconstructed the history of sea level during the past 1000 years using a statistical model driven by four forcing time series reflecting major natural and anthropogenic forcings. Their approach assumes that 1) sea level rise is caused primarily by changes in global ice volume and global ocean heat content, both of which react to changes in forcing with some response time and; 2) there is an equilibrium sea level for a given mean global radiative forcing. They found that sea level has remained within about 20 cm of its present level over the past millennium and that until the 1800s, changes in sea level were driven mostly by natural forcings (solar and volcanic). They note the long lasting effects of volcanic eruptions and found that if no volcanic eruptions had occurred since 1880, then 20th century global sea level would have been 7 cm higher. They conclude that anthropogenic forcing has been the main contributor to SLR since 1900, accounting for more than 70% of the observed SLR. Over the longer period since 1850, anthropogenic forcing was found to account for between 50-70% of SLR. These findings are robust in all their experiments. Of the 18 cm of observed SLR during the 20th century, only 4 cm (± 3 cm) can be attributed to natural climate variability, with the remainder, 14 cm (± 3 cm), being due to the rapid increase of anthropogenic CO2 and other GHGs.
Shindell, D and G. Faluvegi, 2009. Climate response to regional Radiative forcing during the twentieth century. Nature geoscience, volume 2, DOI: 10.1038/NGEO473.
A new study finds that decreasing concentrations of sulphate aerosols and increasing concentrations of black carbon aerosols in mid-latitude regions have substantially contributed to Arctic warming during the last century.
In the past few years, studies have shown that at continental scales, surface temperature changes broadly match the response to forcings from well-mixed greenhouse gases (WM-GHGs). However, on a more regional scale, the sensitivity of the climate to different forcings is not that clear. In this recent study, the authors used the NASA Goddard Institute for Space Studies (GISS) coupled ocean-atmosphere model to look at how surface temperatures in different regions of the world have responded during the 20th century to different forcings (observed and simulated). In addition to natural forcings (solar and volcanic), they used increases in CO2 (representative of the WM-GHGs) and black carbon aerosols and decreases in sulphate (representative of reflective aerosols) and ozone. They found that the extratropical temperatures are much more sensitive to local forcings than tropical temperatures are. Observed 20th century surface temperatures are consistent with these findings. Mid-latitude and polar mean temperatures largely follow local forcings, when the forcing is applied within that latitude band. Looking more closely at the Arctic, the authors found that the temperature changes in this region are, in addition, quite sensitive to emissions coming from lower latitudes. Indeed, their results indicate that the net impact on surface temperatures in the Arctic for the period 1890-2007 has been -0.6°C from tropical aerosols, +0.4°C from mid-latitude aerosols and +0.5°C from Arctic aerosols. Furthermore, during 1976-2007, large changes in mid-latitude emissions have influenced the local Arctic forcing, with estimated surface temperature changes of -0.3°C from tropical aerosols, +0.6°C from mid-latitude aerosols and +0.8°C from Arctic aerosols. The authors conclude that their calculations suggest that black carbon aerosols and tropospheric ozone have contributed ~0.5-1.4°C and ~0.2-0.4°C, respectively, to Arctic warming since 1890.
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