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2006 - 2008 Literature Review Archives - Detection and Attribution

Fyfe, J.C. 2006. Southern Ocean warming due to human influence. Geophys. Res. Lett. 33 L19701,doi:10.1029/2006GL027247.
Using ALACE floats/hydrographic data, the mid-depth temperature trend for the Southern Ocean (35oS to 65oS) was computed showing a mean warming which is nearly double the rate of change in the upper 1000 m of the World Oceans. The author, Environment Canada scientist John Fyfe, investigated this warming trend using a series of ten state-of-the-art climate models. The study found remarkable agreement between the observed warming trend in the mid-depth Southern Ocean and the simulated trend when the models included time-varying changes in anthropogenic greenhouse gases, sulphate aerosols and volcanic aerosols. In the model runs that didn't include volcanic aerosols, the simulated temperature of the mid-depth Southern ocean nearly doubled. This implies that the full effect of human-induced warming may not yet be fully realized.

Gillett, N.P., D.A. Stone, P.A. Stott, T. Nozawa, A.Y. Karpechko, G.C. Hegerl, M.F. Wehner, and P.D. Jones, Attribution of polar warming to human influence, Nature Geoscience, doi:10.1038/ngeo338, 2008.
New study demonstrates that warming in the Arctic and Antarctic can be directly attributed to human influence.
Polar regions have long been expected to warm strongly in response to human-induced climate change, due to the positive feedbacks associated with melting ice and snow. However, while several studies have noted a pronounced Arctic warming over recent decades, polar temperature changes have not previously been formally attributed to human influence either in the Arctic, due to sparse observations and large natural variability, or in the Antarctic, which the IPCC Fourth Assessment Report notes is the only continent where human influence on temperature has not been detected, partly due to insufficient observational coverage. In their study, published in Nature Geoscience, Gillett and colleagues use up-to-date observations and output from four coupled climate models to demonstrate that Arctic and Antarctic land temperature changes are not consistent with internal climate variability or a response to natural climate influences alone and are attributable to human influence on climate. Warming over polar land regions has impacts on biology, indigenous communities, ice sheet mass balance and sea level. These results demonstrate that humans have already caused significant warming in both polar regions.

Hoerling, M., J. Hurrell, J. Eischeid and A. Phillips (2006), Detection and Attribution of Twentieth-Century Northern and Southern African Rainfall change, Journal of Climate 19:16: 3989-4008.
This paper explored the nature and cause(s) of observed drying trends over the Sahel and Southern Africa over the period 1950-1999 using, first of all, statistical analyses of observational data, and then, models, to more directly attribute changes in rainfall to causal factors. They find, confirming a number of previous studies, that the regional drying trends are directly attributable to fluctuations in sea surface temperature (SST), with drought over the Sahel linked to relative cooling of the tropical North Atlantic compared with the tropical South Atlantic, while the Indian Ocean warmth exerts a strong drying effect over southern Africa during austral summer. These findings are confirmed by 80 separate 50-year climate simulations utilizing five different AGCMs - high pattern correlations between the observed trend and modeled trends were found for each AGCM ensemble. To establish if the drying trends could also be the result of GHG forcing, the authors examined the simulated 1950-1999 rainfall trends occurring in 18 AR4 models, using both unforced and forced simulations. The 18-model ensemble response to observed GHG changes during 1950-1999 shows that, for both regions, GHG forcing played little or no role in the observed African drying trends.
In a third step, the authors analyze simulated African rainfall time series with the AR4 models for the 1950-2049 period. They find two coherent but opposite GHG-induced signals during the 21stcentury. The first one is a wet trend over the Sahel (14 out of 18 models) that occurs in lockstep with projected increasing warmth of North Atlantic relative to South Atlantic SSTs. The other signal is a drying trend over Southern Africa, occurring in tandem with the projected Indian Ocean warming. However, both signals are of small amplitude, which suggest that natural variability will continue to be the primary driver of Africa rainfall variations during the next century.

Hoyos, C.D., Agudelo, P.A., Webster, P.K. and Curry, J.A. 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science. ScienceXpress. 10/1126/science.1123560.      Hoyos et al. examined the influence of the trends in wind shear, specific humidity, zonal stretching deformation (i.e. change of zonal wind with longitude) and tropical sea surface temperature (SST) on the number of global hurricanes from 1970 to 2004. The analysis indicates that the current increasing trend in the frequency of category 4 and 5 hurricane is directly linked to the trend of increased SST. Although the other factors showed some influence in the short term trend variability of the number of the category 4 and 5 hurricanes, none of them showed the consistent influence of SST on the long term trend. This finding echoes to the recent studies by Webster (2005) and Emanuel (2005), which demonstrated the strong correlation linking the number of high intensity hurricanes and warmer sea surface temperature (SST) in recent decades.

Lean, J.L. and Rind, D.H. 2008. How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. GRL 35, L18701, doi:10.1029/2008GL034864.
New data-based climate study supports the primary role of human influences on past climate change, but disagrees on the geographical pattern of such change.
Although process-based climate model studies have demonstrated that volcanic eruptions, solar variability, internal climate system variability and human emissions of aerosols and greenhouse gases are the primary factors affecting global climates over the past century, there is continued debate about how much of the past changes in climate can be attributed to each of these factors. A new study, recently published in Geophysical Research Letters, seeks to add to this debate by using a data-based model to compare the most up-to-date estimates for each of these factors with observed trends and patterns of surface temperature changes. Authors find that major ENSO events can enhance global temperatures by 0.2°C, that large volcanic eruptions such as that of Mt. Pinatubo can cause a temporary cooling of 0.3°C, and that global climates warm by about 0.1°C during the peaks of the 11 year solar cycle. They conclude that the net warming effect of these natural factors over the period between 1905 and 2005 is at least an order of magnitude smaller than observed warming over the same time period. Hence most of the observed changes can be attributed to human factors. They also note that their empirical analysis disagrees significantly with climate model studies with respect to some aspects of the regional distribution of changes in climate in response to natural and human forces. For example, while climate model experiments consistently suggest that the temperature response to anthropogenic radiative forcing increases with latitude, their data analysis does not appear to support such a polar amplification. One reason for this could be that climate models underestimate the role of atmospheric circulation as a mechanism influencing the response of climate systems to radiative forcings.

Lockwood, M. and C. Fröhlich (2007), Recent oppositely - directed trends in solar climate forcings and the global mean surface air temperature, Proc. R. Soc. A., doi:10.1098/rspa.2007.1880; 2007, 19 pages.
A new study by European scientists confirms that the Sun is not responsible for recent global warming.
Claims that the Sun is the main cause of recent climate change, rather than increased concentrations of greenhouse gases, are still advanced by a small number of scientists. In a recent analysis, two researchers from Europe compiled solar data (solar flux, cosmic ray intensity) for the past 100 years and looked for correlations between solar variation and global mean temperatures over this time period, using the GISS temperature reconstruction. Specifically, the authors examine solar changes over the 1975-2005 period, in the context of the changes that took place over the 20th century. They found that since 1985, solar radiation has been decreasing and cosmic rays fluxes have been increasing, while surface air temperatures have continued to increase. That is, trends in solar irradiance, sunspot number and cosmic-ray intensity have all been in the opposite direction to that required to explain recent global warming.

Meehl, G.A., J.M. Arblaster and C. Tebaldi. 2007. Contributions of natural and anthropogenic extremes over the United States. Geophysical Research Letters Vol 34, doi:10.1029/2007GL030948.
Most of the changes in temperature extremes over the U.S. from 1950-2000 are likely due to human activity.
The recently published assessment report of Working Group I (Science) of the IPCC established that the evidence for human influences on climate has grown substantially over the past decade and now extends to many aspects of climate besides global average temperature. This paper by Meehl and colleagues is part of this burgeoning body of evidence. The authors show that trends in extreme temperature indices, calculated from simulations of 20th century climate by a number of global climate models (those participating in the Coupled Model Intercomparison Project), generally agree with the observed trends. Decreases in frost days, increases in growing season length, an increase in the number of warm nights and an increase in heat wave intensity have all been observed in the U.S. for the second half of the 20th century. Experiments with two of the models (PCM and CCSM3), run with either natural or anthropogenic forcings alone, are then used to evaluate the relative contribution of natural and human factors to these trends. Agreement between the observed and modeled trends is found only for the simulations with anthropogenic forcing. The natural forcing experiments were unable to generate significant trends in any of the indices over the last half of the 20th century. The authors also establish that the significant observed trends in frost days, growing season length and warm nights lie well outside the range of internal climate variability, indicating a detectable anthropogenic signal for these indices.

Min, S.-K., X. Zhang, F.W. Zwiers, and T. Agnew, 2008. Human influence on Arctic sea ice detectable from early 1990s onwards. GRL, Vol 35, L21701, doi: 10.1029/2008GL035725, 6 pp.
Human-induced climate change has been identified as contributing to the reduction of Arctic sea-ice extent since the early 1990s.
Decline in ice cover extent in the Arctic has been observed since the 1980s, and it has been assumed that human influences have played a role; however, the connection has never been quantified before, until now. A study recently published by Environment Canada scientists has shown that there is a detectable link between anthropogenic climate change and the rapid decline in sea ice extent in the Artic since the early 1990s. The authors compared observed (1953-2006) to simulated (multi-model) sea ice extent, using an optimal fingerprinting technique to investigate when human-induced climate change was first detectable. They found the signal first detectable in the early 1990s; therefore, if modern day, advanced models had been in use back in the 1990s, we would have been able to detect the human influence on sea ice changes at that time. As well, the authors found that this linkage is not just detectable in the summertime minimum ice extent, but is also found in the cooler months from May through December. Models are improving, simulating the major ice dynamics well; however, the models still have some shortcomings and require more work to be able to model characteristics like sea ice thickness better.

Rosenzweig, C., D. Karoly, M. Vicarelli and many authors. 2008 Attributing physical and biological impacts to anthropogenic climate change. Nature Vol 453 15 May 2008, pp353-357; doi:10.1038/nature06937.
A very large number of published studies have shown clearly that physical and biological systems around the world are responding to climate warming. A robust assessment of this database provides strong evidence that such changes can be attributed to anthropogenic causes.
There has been an increasing number of studies of late that have demonstrated both that some aspect of climate has changed significantly beyond that expected from natural variability (detection of change) and that the observed change has resulted from anthropogenic causes (attribution of change). Extending such 'detection and attribution' studies to changes in natural systems is methodologically a much more difficult problem. The first such study to do so using formal detection and attribution methods was published this week in the journal Nature. In this study, Rosenzwieg and colleagues first conduct a meta-analysis of the very large database of observed changes in many different natural systems around the globe. Changes in physical systems include shrinkage of glaciers, melting permafrost, and shifts in spring peak river discharge, while changes in biological systems include phenological shifts such as changes in the timing of emergence, migration or reproduction, as well as change in species distributions and community structure. Their database covers the period 1970 to 2004, with the largest volume of data coming from Europe and North America. There were also a limited number of studies in their database from all other continental areas except Antarctica. At the global scale (i.e. across all the continents) the authors demonstrate that the agreement between the pattern of observed changes in natural systems and the pattern of observed temperature change is "exceptionally unlikely" (P<<0.01) to be due to either natural internal climate variability or natural variability of the systems themselves. Furthermore, they demonstrated that about 90% of the changes in natural systems at the global scale have been in the direction expected as a response to warming. They present similar conclusions at the continental scale for those regions where spatial coverage of the data is sufficient. The authors then make use of results of the IPCC Fourth Assessment Report with respect to attribution of observed changes in temperatures at the continental scale to anthropogenic forcing to conclude that there is now also strong support for attribution of observed impacts in natural systems to human induced climate change.

Santer, B.D., C. Mears, F.J. Wentz, K.E. Taylor, P.J. Gleckler, T.M.L. Wigley, T.P. Barnett, J.S. Boyle, W. Brüggemann, N.P. Gillett, S.A. Klein, G.A. Meehl, T. Nozawa, D.W. Pierce, P.A. Stott, W.M. Washington and M.F. Wehner, 2007. Identification of human-induced changes in atmospheric moisture content. PNAS, vol.104, no.39: 15248-15253.
For the first time, a formal detection and attribution study has found a discernible human influence on changes in atmospheric water vapor. This finding, together with recent related work on continental runoff, zonal rainfall and surface humidity, shows an emerging anthropogenic signal in both the moisture content of Earth's atmosphere and in the cycling of moisture between atmosphere, land and ocean.
In this paper, the authors attempted to identify, in the satellite-borne Special Sensor Microwave Imager (SSM/I) record for the period 1988-2006, a human influence on changes in total atmospheric water vapor (W) over the oceans. First, they looked at the trends in W and found that it increased over the analysis period by 0.41 kg/m2 per decade. Then, they used climate model simulations from 22 climate models (from the Coupled Model Intercomparison Project - CMIP-3) to see if the observed variability of W is reliably captured in 20th century simulations with historical forcings (both natural and anthropogenic). They found that the observed W changes represent a response to a complex mixture of internal climate variability ("noise") (eg. El-Niño events) and external forcing - in other words, the observed trend cannot be explained by internal variability alone. In order to separate out and estimate the contribution of the various external factors and "noise", the authors pooled the results from all 22 models and applied formal detection and attribution analysis methods. They found that the moisture patterns estimated from ALL forcings (natural + anthropogenic) and ANTHRO (anthropogenic only) forcings were very similar. Finally, to quantify the contribution of various factors to the simulated changes in atmospheric water vapour, they used single-forcing experiments performed with two different models and found that the increase in atmospheric water vapour is primarily due to human-caused increases in GHGs.

Stendel, M., I.A. Mogensen, J.H. Christensen, 2006. Influence of various forcings on global climate in historical times using a coupled atmosphere-ocean general circulation model. Climate Dynamics, 26, 1-15.
In an attempt to better simulate historical climate over the past 500 years, three Danish scientists incorporate greater detail of past volcanic forcing into a Global Circulation Model. Events like the Late Maunder Minimum (1675-1715) are detected and attributed to the combined effects of decreased solar irradiation and increased volcanic activities. The cool conditions during the Maunder Minimum in turn shifted the North Atlantic Oscillation towards favourable conditions for arctic ice build-up, leading to further reductions in northern hemisphere temperatures.

Stott, P.A., R.T. Sutton, and D.M. Smith, 2008. Detection and attribution of Atlantic salinity changes. GRL, Vol 35, L21702, doi: 10.1029/2008GL035874, 5 pp.
Changes in salinity in the Mid-Atlantic are attributed to human influences while changes in other ocean basins' salinity cannot yet be.
A study by British scientists has detected human-induced climate change in the Atlantic salinity records. Comparing observed and simulated (using the HadCM3 model) changes in ocean salinity in an optimal detection analysis, the authors have shown that simulations must include anthropogenic climate change to reproduce the observed changes in salinity. There has been a significant increase in the observed salinity in the mid-latitudes (20N-50N). However, the freshening of the sub-polar Atlantic, as observed in 2003, seems to have reversed in recent observations, and therefore does not indicate an anthropogenic link. Changes in this region are in the range of natural variability of the system. Other ocean basins aren't predicted to see noticeable changes in salinity until 2100.

Willett, K.M., N.P. Gillett, P.D. Jones and P.W. Thorne. 2007. Attribution of observed surface humidity changes to human influence. Nature Vol 449 October 11, doi:10.1038/nature06207.
For the first time, scientists have attributed increases in atmospheric humidity mainly to human influence. This adds yet another item to the growing list of climatic variables for which recent changes have been attributed to human causes.
A new homogenized gridded data set of surface humidity was used, in combination with a global climate model, to investigate the causes of increasing specific humidity over the last few decades. Over the period 1973-2002, the observed global mean specific humidity increased by 0.07g/kg per decade. The observed variations in surface specific humidity over the three decades were then compared to simulated changes in humidity using the HadCM3 global climate model forced with either anthropogenic forcings only or natural forcings only. Observed changes were also compared to the sum of the anthropogenic and natural responses (an approach made necessary by the absence of archived surface specific humidity results from combined forcing simulations.) The results provide evidence that the observed upward trend in global mean surface specific humidity is due mainly to anthropogenic forcing. They furthermore conclude that the increase in specific humidity is due to a warming climate under conditions of constant relative humidity. Atmospheric humidity is a key variable in determining many other aspects of climate change of particular significance to humans: the maximum intensity of precipitation, potential maximum intensity of tropical cyclones, and human heat stress, to name a few.

Zhang, X., F.W. Zwiers, and P.A. Stott, 2006: Multimodel multisignal climate change detection at regional scale. J Climate, 19(17), 4294-4307.
Two Environment Canada scientists, Zhang and Zwiers, along with a British scientist from the Hadley Centre, Stott, have clearly detected human-induced climate change at a regional scale in Canada, southern Europe and China. Using four climate models, CGCM1 and CGCM2 from CCCma, and the HadCM2 and HadCM3 from the Hadley centre, this study examined twentieth century temperature changes for 9 different regions: The globe, Northern Hemisphere, Eurasia, North America, Northern Hemisphere land only, the whole band between 30°-70° N, southern Canada, southern Europe and China. They found that only when the combined effects of anthropogenic greenhouse gases and sulphates were incorporated could the models replicate the historical temperature trends and spatial patterns for all the regions examined. Larger areas and ensemble model runs reduced the uncertainty in this detection technique.

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