ECON*2100 Economic Growth and Environmental Quality Climate Change Lectures I. Science Background 1

2 What do we mean by “global warming”? “Since the late 1960s, much of the North Atlantic Ocean has become less salty, in part due to increases in fresh water runoff induced by global warming, scientists say." -Michael Schirber, LiveScience June 29, 2005 “The surface waters of the North Atlantic are getting saltier, suggests a new study of records spanning over 50 years. They found that during this time, the layer of water that makes up the top 400 metres has gradually become saltier. The seawater is probably becoming saltier due to global warming, Boyer says.“ -Catherine Brahic, New Scientist August 23, 2007

3 The Greenhouse Effect The sun sends energy to the Earth The Earth has to send it all back to space

4 The Greenhouse Effect 2 ways energy leaves the Earth’s surface: Convection Radiation

5 Convection Process that creates weather Warm, wet air rises Cool, dry air from above comes down Rain and wind cool the surface

6 Radiation The Earth is like a light bulb The light is too dim for our eyes to see “Infrared Radiation”

Different Proportions Stratosphere: – Mostly radiation Surface & troposphere: – Convection & radiation 7

8 The Greenhouse Effect The sun sends energy to the Earth The Earth has to send it all back to space convection radiation

9 The Greenhouse Effect Some gases absorb infrared light and warm up – Water Vapour – Carbon dioxide (CO2) – Methane – A few others Water Vapour does 95% of the absorption

10 What happens when we burn fossil fuels? The amount of CO2 in the air is going up a little each year

11 The Greenhouse Effect The sun sends energy to the Earth The Earth has to send it all back to space Now some more of the infrared energy is absorbed convection radiation

The problem… While the radiative changes are relatively straightforward to model (linear)… Convection, or fluid dynamics, are very difficult to model (highly nonlinear) Consequently the problem can only be analyzed using empirical approximations 12

What do people agree on? CO2 is a greenhouse gas (infrared absorbing) CO2 concentration in the atmosphere is going up due to fossil fuel use All other things being, this is likely to cause an increase in temperatures Since the early 1800s, the world has warmed Changes due to CO2 are relatively small: large changes require large feedback processes to kick in 13

Where are the disagreements? Feedbacks and amplification mechanisms – Models versus observations Whether natural variability explains some or most of modern climate changes Whether the changes are harmful or not 14

Long Term Context The climate varies naturally on long and short time scales

Long Term Context CO 2 levels vary too

Modern Warming in Context Ice core record 420,000 years 17

The world’s longest thermometer record – Central England – 1650 to

Modern warming: pattern Surface air temperature averages are typically used Also tropospheric air temperature, sea ice, etc. Major regions: – Land – Ocean – Northern Hemisphere – Southern Hemisphere 19

Land record sample size Collapsing sample size 20

Land record sample size Growing bias towards airports 21

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Land and Ocean LandOcean 28

NH and SH Sea Ice 1979 –

Is there a problem? [Present] emission trends put the world plausibly on a path toward 4°C warming within this century. Levels greater than 4°C warming could be possible… Current scientific evidence suggests that even with the current commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100, and a 10 percent chance of 4°C being exceeded as early as the 2070s. – Potsdam Institute for Climate Studies 30

Is there a problem? The only sure thing is what we have so far been able to measure…The warming we have had over the last a 100 years is so small that if we didn’t have had meteorologists and climatologists to measure it we wouldn’t have noticed it at all… It indicates that the climate sensitivity is probably lower than climate models, at least initially adopted. – Lennart Bengtsson, European Weather Centre, Head of Atmospheric and Space Science Institute, Bern 31

Is there a problem? Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia…Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system…It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. 32

Is there a problem? The IPCC iconic statement that there is a high degree of certainty that most of the warming of the past 50 years is due to man’s emissions is, whether true or not, completely consistent with there being no problem. To say that most of a small change is due to man is hardly an argument for the likelihood of large changes. – Richard Lindzen, Meteorologist, MIT 33

Amplification mechanisms Water vapour feedback – As air warms it can hold more moisture – Add a little CO2 – Air warms a little – H2O level rises – Air warms a lot 34

Amplification mechanisms Ice-albedo feedback – “albedo” = reflectivity of surface – Snow-covered ice is highly reflective – Ocean surface is much less so – If polar regions warm, sea ice retreats a bit – Oceans then absorb more solar energy, warming more 35

Feedbacks Without feedbacks, doubling CO2 would warm up surface in a simple climate model by about 1 C 36

Feedbacks IPCC projects CO2 doubling (going from 280 ppm to 560 ppm) will cause 1.5 – 4.5 C warming In other words, feedbacks are strongly positive 37

Feedbacks Most rapid warming is expected to occur in 2 regions: - tropical troposphere - polar surfaces 38

Tropical Troposphere Half the planetary atmosphere Most incoming solar energy enters climate there High humidity = abundant water vapour 55% of amplification effect occurs here in models Temp’s monitored by weather balloons since 1958, satellites since

Tropical Troposphere The problem: Not much warming there despite CO2 levels going up 18% since Current Model Spread Current Observations

Tropical Troposphere Since 1958, CO 2 up by 30% Except for step change in 1978, no trend in mid- troposphere; almost no change in lower troposphere Models predict far too much warming 41

Tropical Troposphere Models BalloonsSatellites 42

Model-Observation Discrepancy IPCC AR5 report 43

Model-Observation Discrepancy IPCC AR5, Box 9.2 Figure 1 Note middle panel begins with volcano and ends with an El Nino 44

Model-Observation Discrepancy ‘The growing divergence between climate model simulations and observations raises the prospect that climate models are inadequate in fundamental ways,’ – Judith Curry, Climatologist, Chair of Earth and Atmospheric Sciences, Georgia Tech 45

Model-Observation Discrepancy “If things continue as they have been, in five years, at the latest, we will need to acknowledge that something is fundamentally wrong with our climate models…A 20-year pause in global warming does not occur in a single modeled scenario. But even today, we are finding it very difficult to reconcile actual temperature trends with our expectations.” – Hans von Storch, Climatologist, GKSS, Germany 46

Conclusions The basic points on which there is wide scientific agreement establish that humans can, and likely do, influence the climate system by raising the atmospheric CO2 fraction On its own this is not enough to prove that the changes pose a problem, or even will be large enough to notice Making that argument requires additional assumptions about positive (amplifying) feedbacks, as represented in computer models of the climate system 47

Conclusions A lot of credible scientists working on the problem believe the positive feedbacks are large enough to pose problems in the years ahead But key discrepancies between models and observations are raising the likelihood that models overstate the feedbacks and the sensitivity of the climate system to CO2 emissions The next 4-5 years will likely suffice to settle this debate 48

Conclusions Science is always about the weight of the evidence. Few things are provable in an absolute sense. One must always be open to surprises, reinterpretations, and the possibility that one has been wrong … Climate change presents us with one of the most difficult of all scientific problems, heavily encumbered with the need to make political and economic decisions long before hard evidence is at hand. – Carl Wunsch, Professor of Physical Oceanography, MIT 49