Download Climate Sensitivity - Earth and Environmental Sciences

V1003 Climate & Society
Due Dec. 3 in class
Homework #5
1. Climate Sensitivity
Show all calculations and do this on your own.
Climate Sensitivity
Climate sensitivity is a measure of the global surface air temperature change for a particular forcing. It is
usually given as a °C change per W/m2 forcing. In the previous homework we examined how surface
temperature might change given prescribed changes in solar irradiance, volcanic eruptions, and CO2.
Here, we consider how climate may change using the past as a guide to the future.
The idea is that there have been well-documented climate changes in the past and we have some idea of
the radiative forcing responsible for the climate change. Radiative forcing is the change in energy balance
(in and out) at the Earth's surface. In this case we will use three known past examples of climate change
to predict a forth – our future.
We know that 20,000 years ago there were large ice sheets over the northern and southern hemisphere
polar regions and ocean and land temperatures cooled. The main causes of the ice age were changes in the
earth's orbit and also changes in albedo and greenhouse gases. We have been able to estimate that the
global surface temperature was cooler by -5 ± 0.8 °C due to a change in radiative forcing of -7 ± 1.3
W/m2.
We also know that global temperatures have warmed up by a certain amount (how much?) since the
preindustrial times (1800s) in response to a known radiative forcing of +1.6 ± 1.0 W/m2. Lastly, we know
the third example: that during preindustrial times there was no temperature change.
Forcing (W/m2)
error
∆Temperature
error
LGM
Pre-Industrial
Today
2100
a) Plot up these three examples on the chart below, and draw your best estimate of a "best fit" line (use a
ruler) running through all data points. Include the error bars for each point as well. (You can use and
show a plot from Microsoft Excel if you'd prefer).
b) Estimate the slope of this line. The slope in this case is Climate Sensitivity – how much global
temperatures change for a given change in radiative forcing. What is the average Climate Sensitivity slope
of these observations?
c) Estimating error is equally important – how well do you know this slope, this Climate Sensitivity?
Based on the uncertainties (errors) in the numbers I've given you estimate the error on the slope of the line
as well, use these to explore the minimum and maximum slope you can get from these observations.
Climate Sensitivity
6
Temperature change (°C)
4
2
0
-2
-4
-6
-8
-6
-4
-2
0
2
4
6
8
Radiative Forcing (W/m2)
d) Now we get to the interesting part. Using the average slope in part (b), what would be the warming
you'd expect in the year 2100 when CO2 will have doubled (or more). Scientists have estimated that the
radiative imbalance due to a doubling of CO2 is +6.7 ± 2.3 W/m2, due principally to increased
greenhouse gases but also changes in albedo and water vapor content.
e) What is the approximate uncertainty of this future warming estimate?
Here are some typical skeptic questions you can answer on your own. Below are some typical assertions
by climate change skeptics that global warming isn’t happening or, if it is, that it’s insignificant. Please
answer to the best of your ability and provide quantitative evidence where possible Please let these be
your own ideas based on what you’ve learned inside or outside this class. (2-3 sentences each question).
f) Most of recent global warming is due to variations in solar irradiance.
g) Climate has changed before and the warming of recent decades is nothing special.
h) The global temperature record from 1880-Present is mostly reflecting urban expansion.
i) Over the last decade global temperatures have been flat or even declining, yet CO2 goes up.
j) The Vostok ice core record shows that temperature leads CO2, not vice versa (so CO2 cannot be a
prime cause).