Cloud feedbacks are an important regulator of Earth's climate, as changes in incoming solar and outgoing longwave radiation are expected to have an influence on clouds, and changes in clouds are expected to have further influences on atmospheric radiation. Such feedbacks are the most prominent source of differences between climate model projections of temperature increases due to increased greenhouse-gas concentrations from one model to another. However, the physics and dynamics which determine the sign and strength of cloud feedbacks are not well known, and it is difficult to measure such feedbacks in the real world and determine whether cloud feedbacks in climate models are represented accurately. In this project, cloud feedbacks accompanying El Nino/Southern Oscillation (ENSO) events are examined in observations and model output, so that ENSO events can be used to assess the fidelity of cloud feedbacks, and a better understanding of the factors determining cloud feedbacks can be achieved in a real-world context. Cloud feedbacks are determined from satellite observations, reanalysis products, and climate models, and attention is devoted to determining which meteorological variables are most important in determining the sign and strength of the feedback.
The work has broader impacts due to the importance of cloud feedbacks as a source of uncertainty in climate model projections of the extent to which the Earth's temperature will increase due to increasing greenhouse gas concentrations. In addition, the work will support and train a graduate student, thereby promoting the next generation of scientists.
The cloud feedback is one of most vexing problems in climate science. We know that clouds cool our climate today. But, as the climate changes, the amount that clouds cool could change. If, as the climate warms, clouds cool less than they do today, that would exacerbate the warming — thereby providing what scientists call a "positive feedback". They could also increase their cooling as the climate warms, which would offset some of the original warming, leading to a negative feedback. Research supported by this NSF grant has measured the change in clouds as the climate varies in response to short-term climate variations, mostly El Niño-La Niña cycles. Much of this work extends previous calculations by the PI of the cloud feedback (Dessler, A determination of the cloud feedback from climate variations over the past decade, Science, 330, DOI: 10.1126/science.1192546, 1523-1527, 2010). Under this grant, we have focused on verifying and extending this work. In Dessler and Loeb, 2013 (Impact of dataset choice on calculations of the short-term cloud feedback, J. Geophys. Res., 118, 2821-2826, doi:10.1002/jgrd.50199, 2013), we investigated impact of different input data sets on the estimate. In Dessler 2013 (Observations of climate feedbacks over 2000-2010 and comparisons to climate models, J. Climate, 26, 333-342, doi: 10.1175/JCLI-D-11-00640.1, 2013), we compared feedbacks from observations to those in climate models, both for short-term climate variations and long-term global warming. In Zhou et al. 2013 (An analysis of the short-term cloud feedback using MODIS data, J. Climate, 26, 4803-4815, doi:10.1175/JCLI-D-12-00547.1, 2013), we re-calculated the feedback, but using a completely different cloud data set. All of these calculations have confirmed (within uncertainties, at least) the Dessler 2010 analysis. We have also used support from this grant to investigate more general issues having to do with climate change, such as the suggestion that clouds are actually causing climate change rather than acting as a feedback (Dessler, A.E., Cloud variations and the Earth's energy budget, Geophys. Res. Lett., 38, L19701, doi: 10.1029/2011GL049236, 2011). Our analysis found no merit in this argument. We also investigated the "fixed anvil temperature" hypothesis, which postulates that cirrus clouds maintain a fixed temperature as the climate warms, and found mixed evidence for this mechanism’s existence (Li, Y., P. Yang, G.R. North, A.E. Dessler: Test of the fixed anvil temperature hypothesis. J. Atmos. Sci., 69, 2317–2328, doi: 10.1175/JAS-D-11-0158.1, 2012). The broader of this research include support of two graduate students. It also includes our efforts at using new media tools to more broadly disseminate our work to the general public. As an example, we produced a youtube video to explain the results of Dessler, 2011 (www.youtube.com/watch?v=C2ngavUkmis) This video has been viewed nearly 6,000 times, which suggests some success in getting our results out beyond the normal academic community.
