PHY392S - Physics of Climate Lecture 23, Page 1

PHY392S Physics of Climate

Lecture 23

Slides based on material from Prof. K. Strong

PHY392S - Physics of Climate Lecture 23, Page 2

Some Definitions (from 1st lecture)

Weather• the fluctuating state of the atmosphere around us, characterized

by the temperature, wind, precipitation, clouds and other weatherelements

Climate• the average weather in terms of the mean and its variability over

a certain time-span and a certain area

“Climate is what we expect, weather is what we get.” Mark Twain

Climate change• statistically significant variations of the mean state of the climate

or of its variability, typically persisting for decades or longer

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Some Questions (from 1st lecture)

• What is your view of climate change?

• Do you have an opinion?

• Do you know enough to have an informed opinion?

• Do you think the climate changing?

• If so, what should we do? If not, what should we do?

• When? How?

• How well do we understand the issue?

• How well do we need to understand the issue?

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Observed Changes in the Climate System

Is the Earth’s climate changing? Yes.Based upon observations of:• temperature (atmosphere and ocean)• precipitation and atmospheric moisture• snow cover, glaciers, extent of land and sea ice• sea level• patterns in atmospheric and oceanic circulation• extreme weather and climate events• overall features of the climate variability

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Some Examples (IPCC 2007)

• Observations since 1961 show that the average temperature ofthe global ocean has increased to depths of at least 3000 m.

• Mountain glaciers and snow cover have declined.• Global average sea level rose at 1.8 mm/year from 1961 to 2003.• Average Arctic temperatures increased at almost twice the global

average rate in the past 100 years.• Satellite data since 1978 show that annual average Arctic sea ice

extent has shrunk by 2.7% / decade (7.4% / decade in summer).• Long-term trends from 1900 to 2005 have been observed in

precipitation amount over many large regions.• More intense and longer droughts have been observed over wider

areas since the 1970s, particularly in the tropics and subtropics.• There is observational evidence for an increase of intense tropical

cyclone activity in the North Atlantic since about 1970, correlatedwith increases of tropical sea surface temperatures.

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Causes of Climate Change

• Increasing atmospheric concentrations of greenhousegases

• Increasing atmospheric concentrations of aerosols(microscopic airborne particles or droplets)

• Land surface properties

• Variations in solar activity

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Human Influence on Climate Change - 1IPCC 1995: “the balance of evidence suggests that there is adiscernible human influence on global climate”IPCC 2001 (TAR): “In the light of new evidence and taking intoaccount the remaining uncertainties, most of the observedwarming over the last 50 years is likely to have been due to theincrease in greenhouse gas concentrations.”IPCC 2007 (AR4): “Most of the observed increase in globallyaveraged temperatures since the mid-20th century is very likelydue to the observed increase in anthropogenic greenhouse gasconcentrations. This is an advance since the TAR’s conclusionthat “most of the observed warming over the last 50 years islikely to have been due to the increase in greenhouse gasconcentrations”. Discernible human influences now extend toother aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and windpatterns.”

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Human Influence on Climate Change - 2

The attribution of climate change to anthropogenic causesinvolves statistical analysis and the careful assessment ofmultiple lines of evidence to demonstrate, within a pre-specifiedmargin of error, that the observed changes are:

• unlikely to be due entirely to internal variability;• consistent with the estimated responses to the given combination

of anthropogenic and natural forcing; and• not consistent with alternative, physically plausible explanations

of recent climate change that exclude important elements of thegiven combination of forcings.

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Human Influence on Climate Change - 3

• It is likely that increases in greenhouse gas concentrations alonewould have caused more warming than observed becausevolcanic and anthropogenic aerosols have offset some warmingthat would otherwise have taken place.

• The observed widespread warming of the atmosphere and ocean,together with ice mass loss, support the conclusion that it isextremely unlikely that global climate change of the past fiftyyears can be explained without external forcing, and very likelythat it is not due to known natural causes alone.

• Warming of the climate system has been detected in changes ofsurface and atmospheric temperatures, temperatures in the upperseveral hundred metres of the ocean and in contributions to sealevel rise.

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Human Influence on Climate Change - 4

• Attribution studies have established anthropogenic contributionsto all of these changes. The observed pattern of troposphericwarming and stratospheric cooling is very likely due to thecombined influences of greenhouse gas increases andstratospheric ozone depletion.

• It is likely that there has been significant anthropogenic warmingover the past 50 years averaged over each continent exceptAntarctica. The observed patterns of warming, including greaterwarming over land than over the ocean, and their changes overtime, are only simulated by models that include anthropogenicforcing. The ability of coupled climate models to simulate theobserved temperature evolution on each of six continentsprovides stronger evidence of human influence on climate thanwas available in the TAR.

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Remaining Uncertainties

• Difficulties remain in reliably simulating and attributing observedtemperature changes at smaller scales.

• On these scales, natural climate variability is relatively largermaking it harder to distinguish changes expected due to externalforcings.

• Anthropogenic forcing is likely to have contributed to changes inwind patterns, affecting extra-tropical storm tracks andtemperature patterns in both hemispheres. However, theobserved changes in the Northern Hemisphere circulation arelarger than simulated in response to 20th century forcing change.

• Uncertainty in the reconstructions of solar and volcanic forcingwhich are based on proxy or limited observational data for all butthe last two decades.

• Cloud feedbacks remain the largest source of uncertainty.

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Climate Models

• Used to simulate the climate responses to different scenarios• Based on physical laws represented by mathematical equations

that are solved using a three-dimensional grid over the globe• Major components of the climate must be represented in sub-

models (atmosphere, ocean, land surface, cryosphere, biosphere),along with the processes that go on within and between them

• Typical resolution of atmospheric models is 250 km in the horizontaland 1 km in the vertical

• Typical resolution of ocean models is 125-250 km in the horizontaland 200-400 m in the vertical

• Equations typically solved for every half hour of a model integration• For processes that take place on much smaller spatial scales than

the model grid, their average effects are approximated by usingphysically based relationships with the larger-scale variables(parameterization)

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Development of Climate Models

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IPCC 2007 Figure SPM.6Projected Temperature Changes

Projected surface temperature changes for the early and late 21st century relative to 1980-1999.• Central and right panels show multi-model average projections for the B1 (top), A1B (middle) and A2 (bottom)

scenarios averaged over 2020-2029 (center) and 2090-2099 (right).• Left panel shows corresponding uncertainties as the relative probabilities of estimated global average warming

from several different model studies for the same periods.

B1

A1B

A2

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The Kyoto Protocol (from 1st lecture)

• “Depending on who you talk to, the Kyoto Protocol is eithera) an expensive, bureaucratic solution to fix a problem that maynot even exist; orb) the last, best chance to save the world from the "time bomb" ofglobal warming.”(CBC - http://www.cbc.ca/news/background/kyoto/index.html)

• These are the extreme positions in the worldwide debate betweengovernments, consumers, environmental groups, scientists, andindustry.

• The impacts of climate variability and the threat of future changesare issues brought to our attention on an almost daily basis.

• A fundamental question is the level of confidence we have in ourknowledge of what controls Earth's climate and what changes willhappen in the future.→ This the topic of our course - the physics of climate.