1. Projections of Future Atlantic Hurricane Activity Hurricane Katrina, Aug. 2005 GFDL model simulation of Atlantic hurricane activity Tom Knutson NOAA / Geophysical Fluid Dynamics Lab Princeton, New Jersey http://www.gfdl.noaa.gov/~tk 1

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3. Climate Change Attribution are observed changes consistent with  expected responses to forcings  inconsistent with alternative explanations Observations All forcing Solar+volcanic Source: IPCC 4th Assessment Report. Used with permission.

4. IPCC Projections of Future Changes in Climate IPCC best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C). Broadly consistent with span quoted for SRES in TAR, but not directly comparable Source: IPCC 4th Assessment Report. Used with permission.

5. There is some evidence that overall Atlantic hurricane activity may have increased since in the 1950s and 60s in association with increasing sea surface temperatures… Source: Kerry Emanuel, J. Climate (2007). PDI is proportional to the time integral of the cube of the surface wind speeds accumulated across all storms over their entire life cycles. 5 Increasing data uncertainty

6. 6 Source: Vecchi et al. Science (2008) Projection 1: Absolute SST ~300% projected increase in Power Dissipation Indirect attribution: CO2  SST  Hurricanes Projection 2: Relative SST Projected change: sign uncertain, +/- 80% No Attribution Supported by dynamical models. Two future projections of Atlantic tropical cyclone power dissipation

7. Example of a regional model that reproduces the interannual variability and trend of Atlantic hurricane counts (1980-2006) 18-km grid model nudged toward large-scale (wave 0-2) NCEP Reanalyses 7 Source: Knutson et al., 2007, Bull. Amer. Meteor. Soc.

8. Late 21 st century projected Atlantic region climate changes: 18-Model CMIP3 ensemble Higher shear Higher potential intensity

9. The 26.5 o C “threshhold temperature” for Atlantic tropical storm formation: a climate dependent threshhold, which may increase to ~28.5 o C by the late 21st century with climate warming. Source: Knutson et al., 2008, Nature Geoscience. 9 Note the decrease in overall number of storms in the warmer climate

10. The regional model simulates increased hurricane rainfall rates in the warmer climate (late 21 st century, A1B scenario) …consistent with previous studies… Present Climate Warm Climate Warm Climate – Present Climate Rainfall Rates (mm/day) Avg. Rainfall Rate Increases: 50 km radius: +37% 100 km radius: +23% 150 km radius: +17% 400 km radius: +10% Average Warming: 1.72 o C Source: Knutson et al., 2008, Nature Geoscience. 10

11. Example of a “double-downscaling” method used to explore frequencies and intensities of Atlantic hurricanes at high resolution Geophysical Fluid Dynamics Laboratory/NOAA

12. Late 21 st Century Climate Warming Projection-- Average of 18 CMIP3 Models (27 Simulated Hurricane Seasons) Source: Bender et al., Science, 2010

13. Conclusions: What can climate science tell us about late 21 st century climate and Atlantic hurricanes? i)Climate science provides a compelling case that human emissions of greenhouse gases have caused much of the long- term global warming over the past 50 yr and 140 yr (IPCC AR4). ii)Climate models project substantial further global warming over the 21 st century, including in the tropical Atlantic. iii)Sea level rise has already begun and further sea level rise is expected to exacerbate storm surge impacts, even if the storms themselves do not change. iv)There is no consensus among current models on future changes in Atlantic tropical storm frequency (+/- 60%). Globally there is a general consensus on a decrease (or little change) in global frequency of tropical storms. v)Most high-resolution models agree that hurricane intensities will increase (range +2 to 11% by 2100) and that hurricanes will have higher rainfall rates (~+20%). In one study, the number of very intense category 4 and 5 Atlantic hurricanes doubles by 2100, though not for all of the climate models used in the study. vi) However, we cannot yet conclude that humans have already caused a detectable change in Atlantic hurricane activity. Note that humans may have already caused changes that are either below the 'detection threshold' or are not yet confidently modeled (e.g., aerosol effects). 13 Decreasing Confidence