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Hawaii
Fiji
1) Why model the climate?
Sachs and Myhrvold: “A Shifting Band of Rain”
Evidence of Past Climate Change
?
Mean annual climatology
Rainfall (Intertropical Convergence Zone
& South Pacific Convergence Zone)
Sachs et al. 2009, Nature Geoscience
Mean annual climatology
Sea surface temperature (West Pacific warm pool)
NOAA’s Climate Prediction Center
Mean annual climatology
Sea surface height (Why are sea levels higher in the west?)
Satellite measured
We are interested in departures from “Normal”
Anomaly = Observation – Climatology
For example,AnomalyMar 2017 = ObservationMar 2017 – ClimatologyMar 1981–2010
That is,If a month is warmer than normal, that month’s anomaly is
positiveIf a month is cooler than normal, that month’s anomaly is
negative
2015–2016 El Niño:Warmest sea surface temperature in equatorial eastern Pacific
2015–2016 El Niño:Trade winds weaken (or reverse)
2015–2016 El Niño:Extreme sea levels (low and high stands)
Guam
2015–2016 El Niño: Shifting rainfall bands
W/m2
Outgoing Longwave Radiation (thermal) emitted from Earth and atmosphere (clouds) out to space
Why do cloud tops emit less?
El Niño-Southern Oscillation (ENSO)
ENSO Bloghttps://www.climate.gov/news-features/blogs/enso
Sea surface temperature drives global atmospheric circulation
Warm air rises, like a hot air balloon
Ocean & Atmosphere are coupled:
For example, warmer Sea Surface Temperature anomalies cause larger wind anomalies (weaker Trade Winds) which cause
further warming.
Positive feedback (“chain reaction”) = Bigger event
2015/16: Strongest El Niño since 1997/98
Sea surface height (satellite measured)
Measuring ENSO
Sea surface temperature index
Trenberth and Fasullo 2013, Earth’s Future
2.4 °CNov 2015
El Niño will likely end soon, La Niña may follow
Climate model forecast
Spring 2016class
Observations
Weak La Niña is over, chance of another El Niño
Climate model forecast
Latest
Observations
Hawaii’s 2016 winter drought was well predicted
Climate model forecast from August 2015 for Dec–Feb
Annamalai et al. 2015, AsiaPacific Issues
Temperature?Precipitation?Humidity?Wind?Clouds?Ice?
Variables
2) How is the climate simulated?Principals of physics, chemistry, biology incorporated into a
mathematical model of climate
https://www.gfdl.noaa.gov/climate-modeling/
Simulate all climate components at each point on earth
https://www.gfdl.noaa.gov/climate-modeling/
2020?
Supercomputers like the Yellowstone machine (National Center for Atmospheric Research)
Many computations are needed!We must solve for values of the variables described by equations
(e.g., conservation of mass, momentum, and heat) over time. But, the equations can not be solved analytically so they must be
discretized in time and space.
MIT OpenCourseWare Global Climate Change
1.5 Trillion Calculations takes only a couple hours on a laptop!
Cost of a simple climate model
Radiative balance between incoming solar shortwave radiation (SWR) and outgoing longwave radiation (OLR)
IPCC Fifth Assessment Report
Figure 1
Main drivers of Climate Change
Representative Concentration Pathway (RCP) Scenarios:How much greenhouse warming?
IPCC Fifth Assessment Report
What causes the wiggles?
End-of-century projections
IPCC Fifth Assessment Report
5 °C ≈ 9 °F Hatching indicates uncertainty
3) What do climate models tell us?
Continued warmingalmost certain
for Hawaii
Rainfall changeuncertainfor Hawaii
Island topography not resolved in global climate models.
Rainfall changeuncertain
for Fiji
Future change depends on how much warming.
Opposing mechanisms yield uncertainty (in some regions)Wet gets wetter
Warmest gets wetter
Widlansky et al. 2012, Nature Climate Change
How will El Niño respond to Climate Change?
“There is high confidence that ENSO will remain the dominant mode of interannual variability with global influences in the 21st century, and due to changes in moisture availability ENSO-induced rainfall variability on regional
scales will intensify.”But,
“There is low confidence in changes in the intensity and spatial pattern of El Niño in a warmer climate.” IPCC
Fifth Assessment Report
Climate model projections
El Niño impacts likely to become more extreme even if sea surface temperature variability remains constant
Cai et al. 2014, Nature Climate Change
Stronger anomalies (contours)required to overcome
large mean gradients (shading)
Smaller mean gradients (shading), therefore atmosphere responds to
weaker anomalies (contours)
Present
Future
For example, likely more future extreme sea level seesaws
Widlansky et al. 2015, Science Advances
But, less tropical cyclones (hurricanes) projected by end of 21st century
RCP8.5 (2071–2100) minus Historical (1976–2005) (% change)
20–30% decrease
Consistent with expected future weakening of tropical circulation (less vertical lift and drying mid-troposphere)
(e.g., Knutson et al. 2010 Nature Geoscience)
Sample climate model tropical cyclone tracks
Discussion: How should scientists and societies respond to climate change uncertainty?
IPCC WG1AR5 Summary for Policy Makers (Table 2)
Sixth Assessment Report (scheduled 2021)
What if we are missing something?Like ice-sheet collapse…
2 mGlobal mean sea level rise
