Join us for this research seminar with Nathan Gillett, co-hosted by the Climate Research Lab, Simon Fraser University and the Pacific Institute for Climate Solutions.
The Paris Agreement commits ratifying parties to pursuing efforts to limit the global temperature increase to 1.5 °C. Carbon budgets consistent with remaining below 1.5 °C global warming reported in the Working Group I contribution to the Fifth Assessment Report of the IPCC are directly based on responses from CMIP5 earth system models, which, on average, tend to warm more than observations in response to historical CO2 emissions and other forcings. Following the IPCC approach and calculating budgets relatively to 1861-1880, and then subtracting emissions to date yields a median remaining 1.5 °C carbon budget in 2015 of 55 PgC, which can be compared with annual emissions of 10.6 PgC yr-1 in the same year. However, calculating carbon budgets and temperature changes relative to the last decade (2006-2015) eliminates uncertainties related to model spread and model-observation differences over the historical period, and increases the median remaining 1.5 °C carbon budget, to 198 PgC. While it might be thought that uncertainties in projected budgets could be further constrained by choosing a subset of models with historical warming most consistent with observations, we demonstrate that subsetting CMIP5 models in this way does not substantially change calculated CO2 emissions budgets. We further explore the substantial influence of non-CO2 forcings including land use change on this budget in earth system model simulations, and demonstrate their combined effect is primarily through enhanced climate warming, rather than through induced changes in the carbon cycle. Overall, while limiting median projected global warming to below 1.5 °C is undoubtedly challenging, our results indicate it is not impossible as might be inferred from IPCC carbon budgets.
Bio: Nathan Gillett holds a PhD in atmospheric physics from the University of Oxford. After his doctorate, Nathan worked as a post-doc at the University of Victoria in Canada on the detection and attribution of climate change, before being appointed as a lecturer then reader at the Climatic Research Unit of the University of East Anglia. In 2008, Nathan returned to Canada to work as a research scientist at the Canadian Centre for Climate Modelling and Analysis (CCCma). He was appointed manager of CCCma in 2014, where he oversees the development and application of Canada’s earth system model. His primary research interests are in detection and attribution of climate change, and the influence of stratospheric ozone depletion on climate. He served as a lead author of the IPCC Fourth and Fifth Assessment Reports and of the 2014 WMO/UNEP Ozone Assessment. He is an adjunct professor in SFU’s Department of Geography.