Climate Change

hp://www.independent.co.uk What determines the temperature of the earth?

The earth’s interior is very hot, but the mantle is a good insulator. The amount of heat seeping out of the interior is very small compared to the energy received from the sun.

How do we measure heat ﬂow? The temperature gradient in the upper part of the crust is determined by directly measuring temperatures at diﬀerent elevaons in boreholes.

Heat ﬂow at the surface of the earth. Note how the areas of highest heat ﬂow follow the mid-ocean ridges. The largest areas of measurement uncertainty are along the very crests of the ridges and under the Greenland and Antarcc ice caps.

Davies and Davies, Solid Earth, vol 1, p 5-24, 2010. What determines the temperature of the earth?

The earth’s interior is very hot, but the mantle is a good insulator. The amount of heat seeping out of the interior is very small compared to the energy received from the sun.

The principal factor controlling the temperature of the earth is the fracon of light it absorbs from the sun.

We have good pictures of the earth taken from space, so we know how much light the earth reﬂects. So, we know how much power the sun provides to the earth. It is a big number, but the earth is big. Everything emits light

In the previous chapter we learned that everything with a temperature emits a spectrum of electromagnec radiaon, or “light”. The wavelength of the peak of this distribuon is given by

L = 3000/T

Where T is in Kelvin and L is in micro-meters.

The surface temperature of the sun is 6000 K, so the peak wavelength emied is 0.5 micro-meters, which is right in the middle of the visible spectrum.

The average surface temperature of the earth is approximately 300 Kelvin, so the peak wavelength emied is 10 micro-meters, which is in the infrared.

visible light infrared light Equilibrium

If the earth’s temperature is constant (equilibrium) then its temperature should be whatever it takes so the infrared light emied by the earth has the same total power as what the earth absorbs from the sun.

infrared light is emied

some of the sun’s light is reﬂected visible light

Power absorbed from sun equals power radiated in all direcons. Oops

If we set the power absorbed by the earth from the sun equal to the power that a solid body would radiate due to its temperature, we can calculate that temperature.

This calculaon has been done and the answer one gets is 255 Kelvin (which is -18 Cengrade or -1 Fahrenheit).

But that is not correct. The average temperature of the earth is 289 Kelvin (16 C, or 61 F). What is wrong?

Oops

This calculaon has been done and the answer one gets is 255 Kelvin (which is -18 Cengrade or -1 Fahrenheit).

But that is not correct. The average temperature of the earth is 289 Kelvin (16 C, or 61 F). What is wrong?

The earth has an atmosphere!

With no atmosphere, these would be equal

And the earth would be fairly cold. The main gasses in the atmosphere (nitrogen, oxygen, some argon) are transparent to visible light and also to infrared light. They go right through the atmosphere.

However, other gases which are present in the atmosphere such as water vapor, carbon dioxide, methane, and others, strongly absorb infrared light.

This absorpon of visible and infrared light by these gasses is extremely well characterized by laboratory experiments. No doubts there. --Wikipedia Atmosphere absorbs infrared These two have to balance from the ground and emits it both into space and back to the ground

And so the earth is much warmer. The actual situaon is more complicated, but can be modeled well. Some visible Some infrared light is is transmied reﬂected by by the clouds. atmosphere.

And so the earth is much warmer. The process is straighorward to understand quantavely

hps://www.skepcalscience.com This is prey basic science. Sciensts would say they have an good theorecal understanding of the earth’s equilibrium temperature.

It is clear that for current condions, increasing water vapor, carbon dioxide, or methane, will lead to a higher equilibrium temperature.

It would be very stupid to argue about this. It would be like arguing about whether or not designing a car with a lower drag coeﬃcient would reduce gasoline consumpon. This is prey basic science. Sciensts would say they have an good theorecal understanding of the earth’s equilibrium temperature.

It is clear that for current condions, increasing water vapor, carbon dioxide, or methane, will lead to a higher equilibrium temperature.

It would be very stupid to argue about this. It would be like arguing about whether or not designing a car with a lower drag coeﬃcient would reduce gasoline consumpon.

So, what is the trend? Measurements. It is actually straighorward to measure the amount of CO2 in the atmosphere with chemical sensors. The Naonal Oceanic and Atmospheric Administraon has maintained such a sensor near the summit of Mauna Loa in Hawaii since 1956.

Measurements.

The seasonal change is due to the seasonal change in the uptake of CO2 by the world’s land vegetaon, because most of the world’s land vegetaon is in the northern hemisphere. Further back

The record extends back to 1956.

Even Further back

--From your book Much Further back

The most direct method for measuring atmospheric carbon dioxide concentraons for periods before instrumental sampling is to measure bubbles of air (ﬂuid or gas inclusions) trapped in the Antarcc or Greenland ice sheets.

The most widely accepted of such studies come from a variety of Antarcc cores and indicate that atmospheric CO2 concentraons were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.

thousands of years ago

--Wikipedia This is quite aenon geng

From your book: Other measurements (not shown) tell us that the carbon dioxide level now is higher than it has been at any me in the last 20 million years. That fact is not disputed; it is astonishing but not surprising. We know how much carbon we are burning, and that is plenty to account for the increase. (Some of the CO2 dissolves in the oceans, making them more acid, and some is taken up by increased biomass.)

This is quite aenon geng

By the way, it is also accepted that 600 million years ago the level of CO2 in the atmosphere was 10-20 mes higher than it is now. And it is equally accepted that there was almost no frozen water on the planet. The poles were fully melted. So, what is the temperature doing?

Physics teaches us that increasing carbon dioxide in the atmosphere must cause the earth’s temperature to rise.

Measurements show that CO2 levels are increasing.

So, what is temperature doing? So, what is the temperature doing?

Physics teaches us that increasing carbon dioxide in the atmosphere must cause the earth’s temperature to rise.

Measurements show that CO2 levels are increasing.

So, what is temperature doing?

First, let’s ask how we know.

So, what is the temperature doing?

First, let’s ask how we know.

The simplest answer is that people have been keeping records of temperature all over the planet for a long me.

“The instrumental temperature record provides the temperature of Earth's climate system from the historical network of in situ measurements of surface air temperatures and ocean surface temperatures. Data are collected at thousands of meteorological staons, buoys and ships around the globe. The longest-running temperature record is the Central England temperature data series, that starts in 1659. The longest-running quasi-global record starts in 1850. In recent decades more extensive sampling of ocean temperatures at various depths have begun allowing esmates of ocean heat content but these do not form part of the global surface temperature datasets.” --Wikipedia So, what is the temperature doing?

Instrumental record:

Global mean surface temperature change from 1880 to 2016, relave to the 1951–1980 mean. The black line is the global annual mean and the red line is a smoothing. The blue uncertainty bars show a 95% conﬁdence limit. Source: NASA GISS So, what is the temperature doing?

Instrumental record: Pay aenon to 1985

We also have satellites measuring the earth’s temperature.

This sounds like it would be highly reliable and straighorward, since a satellite can see the enre earth. But it is actually challenging.

Satellites do not measure temperature directly, but rather radiance at various wavelength from which temperature can be inferred. Also, the satellite data are not homogeneous, but are constructed from a series of satellites with similar but not idencal sensors. So, what is the temperature doing?

We also have satellites measuring the earth’s temperature. So, what is the temperature doing?

Other analyses are less striking, but sll quite together. So, what is the temperature doing?

Temperatures measured on land and at sea for more than a century, as well as more recent satellite data, show that Earth's globally averaged surface temperature is rising.

Since 1970, global surface temperature rose at an average rate of about 0.17°C (around 0.3° Fahrenheit) per decade.

This is more than twice as fast as the 0.07°C per decade increase observed for the enre period of recorded observaons (1880-2015).

The average global temperature for 2016 was 0.94°C (1.69°F) above the 20th century average of 13.9°C (57.0°F), surpassing the previous record warmth of 2015 by 0.04°C (0.07°F). So, what is the temperature doing?

According to the oﬃcial 2016 global report from NOAA's Naonal Centers for Environmental Informaon:

[2016] marks the ﬁh me in the 21st century a new record high annual temperature has been set (along with 2005, 2010, 2014, and 2015) and also marks the 40th consecuve year (since 1977) that the annual temperature has been above the 20th century average.

To date, all 16 years of the 21st century rank among the seventeen warmest on record (1998 is currently the eighth warmest.) The ﬁve warmest years have all occurred since 2010. Correlaon does not prove causaon

Fact: The amount of CO2 in the atmosphere has increased from 280 ppm to the present 407 ppm (45% increase) in the last few hundred years, and 325 to 407 ppm (25% increase) in the last 50 years.

Fact: In the last 30 years, the temperature of the earth has gone up by 0.5 C, or about 1 degree F.

This does not PROVE that the increase in CO2 caused the increase in temperature.

Correlaon does not prove causaon

Fact: The amount of CO2 in the atmosphere has increased from 280 ppm to the present 407 ppm (45% increase) in the last few hundred years, and 325 to 407 ppm (25% increase) in the last 50 years.

Fact: In the last 30 years, the temperature of the earth has gone up by 0.5 C, or about 1 degree F.

This does not PROVE that the increase in CO2 caused the increase in temperature.

Science never proves anything.

But, we understand how the increase in CO2 would result in the temperature increase, and the results agree with physical laws that work in many disparate systems.

Perhaps more importantly, no one has an alternave explanaon that holds up under scruny. What is causing these changes?

Given the ming, the most obvious explanaon is humans burning fossil fuels.

However, this idea has been challenged. It is polically charged. What do we actually know?

First, the anthropogenic idea is certainly plausible: What is causing these changes?

Given the ming, the most obvious explanaon is humans burning fossil fuels.

However, this idea has been challenged. It is polically charged. What do we actually know?

First, the anthropogenic idea is certainly plausible:

hps://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#/media/File:Global_Carbon_Emissions.svg What is causing these changes?

Given the ming, the most obvious explanaon is humans burning fossil fuels.

However, this idea has been challenged. It is polically charged. What do we actually know?

First, the anthropogenic idea is certainly plausible:

The increase in CO2 in the atmosphere accounts for only about half of all the CO2 released from burning fuels.

hps://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#/media/File:Global_Carbon_Emissions.svg The increase in CO2 in the atmosphere accounts for only about half of all the CO2 released from burning fuels.

Where is the other half?

The natural sinks are: • Absorpon of carbon dioxide by the oceans via physicochemical and biological processes • Photosynthesis by terrestrial plants

There is real concern for how much more CO2 the oceans can absorb. Human Carbon Emission

hps://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#/media/File:Global_Carbon_Emissions.svg Human Carbon Emission

hps://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions Human Carbon Emission

hps://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions A boom line

This is just basic fact: over the last 200 years, human acvies have added about 500 billion metric tons of carbon dioxide to the atmosphere. Today, humans add about 100 million metric tons of carbon dioxide to the atmosphere every day.

Physics teaches us that increased CO2 in the atmosphere will cause the earth’s average temperature to increase.

There is overwhelming scienﬁc evidence that:

The amount of carbon dioxide in the atmosphere has increased from about 278 parts per million (ppm) in 1800 to about 410 ppm today. Today's carbon dioxide levels much higher than at any other me in the last 800,000 years.

Earth's average surface temperature has risen by about 1.5°F (0.85°C) since 1880.

hps://www.climate.gov Evidence Claim: The amount of carbon dioxide in the atmosphere has increased from about 278 parts per million (ppm) in 1800 to about 410 ppm today. Today's carbon dioxide levels much higher than at any other me in the last 800,000 years.

Evidence: CO2 is a simple molecule to detect, and highly accurate chemical sensors have been recording the level in the atmosphere of 60 years. Trapped bubbles of gas frozen in ice provide a record from before this me.

Claim: Earth's average surface temperature has risen by about 1.5°F (0.85°C) since 1880.

Evidence: Thousands of weather staons worldwide—over land and ocean—have been recording daily high and low temperatures for many decades and, in some locaons, for more than a century. When different scienfic and technical teams in different U.S. agencies (e.g., NOAA and NASA) and in other countries (e.g., the U.K.'s Hadley Centre) average these data together, essenally the same results are found.

Another boom line

Physics teaches us that increased CO2 in the atmosphere will cause the earth’s average temperature to increase.

There is overwhelming scienﬁc evidence that:

Earth's average surface temperature has risen by about 1.5°F (0.85°C) since 1880.

hps://www.climate.gov Another boom line There is overwhelming scienﬁc evidence that:

Earth's average surface temperature has risen by about 1.5°F (0.85°C) since 1880.

There is a preponderance of evidence that human acvies, namely burning of fossil fuels, is responsible for the increased CO2 and thus the increased temperature.

The increased amount of CO2 in the atmosphere is consistent with (about half of ) what humans have released. Sciensts have not been able to idenfy another source for the CO2.

The increased warming is consistent with atmospheric models predicng how much warming to expect based on increased CO2.

hps://www.climate.gov Close to home

Many sciensts predicted these changes before they happened. Even the science ﬁcon author Isaac Asimov wrote about this well before the data had become reliable.

Once scienst was Gilbert Plass.

In 1953, Plass wrote: "At its present rate of increase, the CO2 in the atmosphere will raise the earth's average temperature 1.5° Fahrenheit every 100 years. ... for centuries to come, if man's industrial growth connues, the earth's climate will connue to grow warmer."

From 1956 onwards he published a series of papers on the topic, partly based on advanced calculaons of the absorpon of infrared radiaon, and he made use of early electronic computers. Plass predicted that a doubling of CO2 would warm the planet by 3.6 °C, that CO2 levels in 2000 would be 30% higher than in 1900 and that the planet would be about 1 °C warmer in 2000 than in 1900. Close to home

Many sciensts predicted these changes before they happened. Even the science ﬁcon author Isaac Asimov wrote about this well before the data had become reliable.

Once scienst was Gilbert Plass.

Plass was head of the physics department at Texas A&M University from 1968 unl around 1975 (about when I started as an undergraduate). Some popular misconcepons

hps://www.climate.gov Some popular misconcepons

Claim: Volcanoes emit more carbon dioxide than humans.

Reality: Human acvies emit more than 100 mes more carbon dioxide than volcanoes do in a typical year.

hps://www.climate.gov Some popular misconcepons

Claim: Warming stopped aer 1998, although CO2 emissions have greatly increased since then.

Reality: No, the globe did not stop warming aer 1998. While 1998 was one of the ten warmest years on record, the other nine warmest years have all occurred aer 1998.

It's true that humans have released more carbon dioxide into the atmosphere from 1998 to 2012 than in any other 15-year period in history, and it's true there was a slowdown in the rate of global warming during that me. Most of the excess heat (>80%) from global warming has been going into the ocean. The point is global warming didn't stop over the last decade; most of the warming happened in the ocean rather than in the lower atmosphere.

Sciensts are always reassessing their esmates of climate sensivity based on observed changes in temperature and ocean heat content. It's too early to conclude that the climate system isn't as sensive to carbon dioxide as sciensts thought, though that possibility is being acvely researched.

hps://www.climate.gov Some popular misconcepons

Claim: The warming we see is just a natural return to equilibrium aer an unusually cool period around 1600 (oen called the “Lile Ice Age”).

Reality: This is unlikely. Here is what several independent temperature reconstrucons show:

Examples of proxies include ice cores, tree rings, sub-fossil pollen, boreholes, corals, lake and ocean sediments, and carbonate speleothems.

hps://en.wikipedia.org/wiki/Global_warming Some popular misconcepons

Claim: The sun’s output has increased slightly and this is the cause of the observed warming.

Reality: There has been no signiﬁcant net change in the sun's energy output from the late 1970s to the present, which is the period of the most rapid warming. If the sun had intensiﬁed its energy output then all layers of Earth's atmosphere would warm. But such warming hasn't been observed. Rather, warming has occurred in the lower atmosphere (troposphere) and cooling in the upper atmosphere (stratosphere)—which is exactly what would be expected if the warming was due to an increase in heat-trapping gases near the surface.

This evidence from temperature records is regarded as a “smoking gun” linking today's global warming to the increase in heat-trapping gases in the lower atmosphere.

hps://www.climate.gov Where are we going? Earth is likely to warm more.

Remember the ice-core record from the last 800,000 years. Just as CO2 concentraon can be measured in the air bubbles, so can the rao of oxygen-16 to oxygen-18. Water with the heavier isotope (18O) condenses more readily as temperatures decrease and falls as precipitaon, while the lighter isotope (16O) can fall in even colder condions.

Note the correlaon between CO2 and temperature:

thousands of years ago Earth is likely to warm more.

We were already at the end of a warming phase when CO2 hit 280 ppm before fossil fuels started being burned.

Temperature was already peaking

Before CO2 rose up to here!

thousands of years ago Orbital variaons (Milankavich cycles) How much more?

The ﬁrst thing to keep in mind is that even if carbon emissions are curtailed, the concentraon in the air will stay a while.

The “lifeme” of increased CO2 in the atmosphere is diﬃcult to model. Current best models predict somewhere between 30 and 100 years.

Journal of Geophysical Research, Vol. 110, D14105. IPCC

IPCC is the Intergovernmental Panel on Climate Change.

From your book: “The IPCC is important; you can’t talk about climate change without knowing it, any more than you can talk about world aﬀairs without knowing the leers UN.

“The IPCC aempts to do the impossible: reach a consensus among hundreds of sciensts, diplomats, and policians. As a result, its conclusions are oen muted and mixed, but its reports contain a wealth of data that help everyone evaluate what is going on. The IPCC shared the 2007 Nobel Peace Prize with former Vice President Al Gore. You need to know the inials. Memorize them: IPCC. You don’t have to remember what the leers stand for.

“It is important to know that there is a scienﬁc consensus—even if not everyone agrees with it. It is also important to know what that consensus actually is, since people on both sides will exaggerate it or distort it and mislead you into thinking that they are describing the IPCC conclusions.” Summary: A report of Working Group I of the Intergovernmental Panel on Climate Change

Summary for Policymakers

Drafting Authors: Richard B. Alley, Terje Berntsen, Nathaniel L. Bindoff, Zhenlin Chen, Amnat Chidthaisong, Pierre Friedlingstein, Jonathan M. Gregory, Gabriele C. Hegerl, Martin Heimann, Bruce Hewitson, Brian J. Hoskins, Fortunat Joos, Jean Jouzel, Vladimir Kattsov, Ulrike Lohmann, Martin Manning, Taroh Matsuno, Mario Molina, Neville Nicholls, Jonathan Overpeck, Dahe Qin, Graciela Raga, Venkatachalam Ramaswamy, Jiawen Ren, Matilde Rusticucci, Susan Solomon, Richard Somerville, Thomas F. Stocker, Peter A. Stott, Ronald J. Stouffer, Penny Whetton, Richard A. Wood, David Wratt

Draft Contributing Authors: J. Arblaster, G. Brasseur, J.H. Christensen, K.L. Denman, D.W. Fahey, P. Forster, E. Jansen, P.D. Jones, R. Knutti, H. Le Treut, P. Lemke, G. Meehl, P. Mote, D.A. Randall, D.A. Stone, K.E. Trenberth, J. Willebrand, F. Zwiers

This Summary for Policymakers should be cited as: IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

hps://www.ipcc.ch/publicaons_and_data/ar4/wg1/en/spmsspm-projecons-of.html IPCC Scenarios We don’t know what is going to happen for many reasons, but most importantly because we don’t know what people are going to do!

Instead we try to predict outcomes based on diﬀerent “scenarios” or collecons of condions that may obtain.

A1. The A1 storyline and scenario family describes a future world of very rapid economic growth, global populaon that peaks in mid-century and declines thereaer, and the rapid introducon of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interacons, with a substanal reducon in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternave direcons of technological change in the energy system. The three A1 groups are disnguished by their technological emphasis: fossil-intensive (A1FI), non-fossil energy sources (A1T) or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one parcular energy source, on the assumpon that similar improvement rates apply to all energy supply and end use technologies).

A2. The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self- reliance and preservaon of local idenes. Ferlity paerns across regions converge very slowly, which results in connuously increasing populaon. Economic development is primarily regionally oriented and per capita economic growth and technological change more fragmented and slower than other storylines.

B1. The B1 storyline and scenario family describes a convergent world with the same global populaon, that peaks in mid-century and declines thereaer, as in the A1 storyline, but with rapid change in economic structures toward a service and informaon economy, with reducons in material intensity and the introducon of clean and resource-eﬃcient technologies. The emphasis is on global soluons to economic, social and environmental sustainability, including improved equity, but without addional climate iniaves.

B2. The B2 storyline and scenario family describes a world in which the emphasis is on local soluons to economic, social and environmental sustainability. It is a world with connuously increasing global populaon, at a rate lower than A2, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented towards environmental protecon and social equity, it focuses on local and regional levels.

An illustrave scenario was chosen for each of the six scenario groups A1B, A1FI, A1T, A2, B1 and B2. All should be considered equally sound. According to the IPCC, all SRES scenarios are considered “neutral”. None of the SRES scenarios project future disasters or catastrophes, e.g., wars and conﬂicts, and/or environmental collapse.

The scenarios are not described by the IPCC as represenng good or bad pathways of future social and economic development. Family A1 A2 B1 B2

Scenario Group A1C A1G A1B A1T A2 B1 B2

Populaon growth low low low low high low medium

GDP growth very high very high very high very high medium high medium

Energy use very high very high very high high high low medium

Land- use changes low-medium low-medium low low medium/high high medium

Resource high high medium medium low low medium availability Pace and direcon rapid rapid rapid rapid slow medium medium of technological eﬃciency & "dynamics as change favoring coal oil & gas balanced non-fossils regional dematerializaon usual"

A1: kinda got our act together B1: Star Trek level opmism. polically, but sll… C: Coal B2: Everything set to medium G: Oil and gas B: Balanced T: Renewables

A2: Burn baby burn!

Family A1 A2 B1 B2

Scenario Group A1C A1G A1B A1T A2 B1 B2

Populaon growth low low low low high low medium

GDP growth very high very high very high very high medium high medium

Energy use very high very high very high high high low medium

Land- use changes low-medium low-medium low low medium/high high medium

A1: kinda got our act together B1: Star Trek level opmism. polically, but sll… C: Coal B2: Everything set to medium G: Oil and gas B: Balanced T: Renewables

A2: Burn baby burn!

Turns out that A1C and A1G (called A1FI) is worse than A2 for emission and warming Summary for Policymakers

MULTI-MODEL AVERAGES AND ASSESSED RANGES FOR SURFACE WARMING

Burn, baby, burn 3.6 °C

Star Trek 1.8 °C

Figure SPM.5. Solid lines are multi-model global averages of surface warming (relative to 1980–1999) for the scenarios A2, A1B and B1, shown as continuations of the 20th century simulations. Shading denotes the ±1 standard deviation range of individual model annual averages. The orange line is for the experiment where concentrations were held constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and likely ranges in the grey bars includes the AOGCMs in the left part of the ﬁ gure, as well as results from a hierarchy of independent models and observational constraints. {Figures 10.4 and 10.29}

TAR model average for 2090–2099. The ranges are the upper ranges of sea level rise for SRES scenarios narrower than in the TAR mainly because of improved shown in Table SPM.3 would increase by 0.1 to 0.2 m. information about some uncertainties in the projected Larger values cannot be excluded, but understanding of contributions.15 {10.6} these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level • Models used to date do not include uncertainties in rise. {10.6} climate-carbon cycle feedback nor do they include the full effects of changes in ice sheet fl ow, because a • Increasing atmospheric carbon dioxide concentrations basis in published literature is lacking. The projections lead to increasing acidifi cation of the ocean. Projections include a contribution due to increased ice fl ow from based on SRES scenarios give reductions in average Greenland and Antarctica at the rates observed for 1993 global surface ocean pH16 of between 0.14 and 0.35 to 2003, but these fl ow rates could increase or decrease units over the 21st century, adding to the present in the future. For example, if this contribution were to decrease of 0.1 units since pre-industrial times. {5.4, grow linearly with global average temperature change, Box 7.3, 10.4}

15 TAR projections were made for 2100, whereas projections in this report are for 2090–2099. The TAR would have had similar ranges to those in Table SPM.3 if it had treated the uncertainties in the same way. 16 Decreases in pH correspond to increases in acidity of a solution. See Glossary for further details. 14 Each scenario has its own uncertainty

These are diﬀerent models of the Burn, Baby, Burn scenario Summary for Policymakers

• Sea ice is projected to shrink in both the Arctic and There is now higher conﬁ dence in projected patterns Antarctic under all SRES scenarios. In some projections, of warming and other regional-scale features, arctic late-summer sea ice disappears almost entirely including changes in wind patterns, precipitation by the latter part of the 21st century. {10.3} and some aspects of extremes and of ice. {8.2, 8.3, 8.4, 8.5, 9.4, 9.5, 10.3, 11.1} • It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent. {10.3} • Projected warming in the 21st century shows scenario- Based on a range of models, it is likely that future independent geographical patterns similar to those • tropical cyclones (typhoons and hurricanes) will observed over the past several decades. Warming is become more intense, with larger peak wind speeds expected to be greatest over land and at most high and more heavy precipitation associated with ongoing northern latitudes, and least over the Southern Ocean increases of tropical sea surface temperatures. There is and parts of the North Atlantic Ocean (see Figure less conﬁ dence in projections of a global decrease in SPM.6). {10.3} These numbers are AVERAGE warming. numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in • Snow cover is projected to contract. Widespread some regions is much larger than simulated by current increases in thaw depth are projected over most It will not be uniform. models for that period. {9.5, 10.3, 3.8} permafrost regions. {10.3, 10.6}

PROJECTIONS OF SURFACE TEMPERATURES

Star trek

Act together

BBB

Figure SPM.6. Projected surface temperature changes for the early and late 21st century relative to the period 1980–1999. The central and right panels show the AOGCM multi-model average projections for the B1 (top), A1B (middle) and A2 (bottom) SRES scenarios averaged over the decades 2020–2029 (centre) and 2090–2099 (right). The left panels show corresponding uncertainties as the relative probabilities of estimated global average warming from several different AOGCM and Earth System Model of Intermediate Complexity studies for the same periods. Some studies present results only for a subset of the SRES scenarios, or for various model versions. Therefore the difference in the number of curves shown in the left-hand panels is due only to differences in the availability of results. {Figures 10.8 and 10.28}

15 Summary for Policymakers

(A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C). Continued greenhouse gas emissions at or above Although these projections are broadly consistent with current rates would cause further warming and the span quoted in the TAR (1.4°C to 5.8°C), they are induce many changes in the global climate system not directly comparable (see Figure SPM.5). The Fourth during the 21st century that would very likely be Assessment Report is more advanced as it provides best larger than those observed during the 20th century. estimates and an assessed likelihood range for each of {10.3} the marker scenarios. The new assessment of the likely ranges now relies on a larger number of climate models of increasing complexity and realism, as well as new • Advances in climate change modelling now enable information regarding the nature of feedbacks from the best estimates and likely assessed uncertainty ranges to carbon cycle and constraints on climate response from be given for projected warming for different emission observations. {10.5} scenarios. Results for different emission scenarios are provided explicitly in this report to avoid loss of this • Warming tends to reduce land and ocean uptake of policy-relevant information. Projected global average atmospheric carbon dioxide, increasing the fraction of surface warmings for the end of the 21st century anthropogenic emissions that remains in the atmosphere. (2090–2099) relative to 1980–1999 are shown in Table For the A2 scenario, for example, the climate-carbon SPM.3. These illustrate the differences between lower cycle feedback increases the corresponding global and higher SRES emission scenarios, and the projected average warming at 2100 by more than 1°C. Assessed warming uncertainty associated with these scenarios. upper ranges for temperature projections are larger {10.5} than in the TAR (see Table SPM.3) mainly because the broader range of models now available suggests • Best estimates and likely ranges for global average stronger climate-carbon cycle feedbacks. {7.3, 10.5} surface air warming for six SRES emissions marker scenarios are given in this assessment and are shown • Model-based projections of global average sea level in Table SPM.3. For example, the best estimate for rise at the end of the 21st century (2090–2099) are the low scenario (B1) is 1.8°C (likely range is 1.1°C shown in Table SPM.3. For each scenario, the midpoint to 2.9°C), and the best estimate for the high scenario of the range in Table SPM.3 is within 10% of the Sea level rise. Table SPM.3. Projected global average surface warming and sea level rise at the end of the 21st century. {10.5, 10.6, Table 10.7}

Temperature Change Sea Level Rise (°C at 2090-2099 relative to 1980-1999)a (m at 2090-2099 relative to 1980-1999) Best Likely Model-based range excluding future Case estimate range rapid dynamical changes in ice ﬂ ow

Constant Year 2000 concentrationsb 0.6 0.3 – 0.9 NA B1 scenario 1.8 1.1 – 2.9 0.18 – 0.38 ST A1T scenario 2.4 1.4 – 3.8 0.20 – 0.45

B2 scenario 2.4 1.4 – 3.8 0.20 – 0.43

A1B scenario 2.8 1.7 – 4.4 0.21 – 0.48

A2 scenario 3.4 2.0 – 5.4 0.23 – 0.51 BBB A1FI scenario 4.0 2.4 – 6.4 0.26 – 0.59

Table notes: a These estimates are assessed from a hierarchy of models that encompass a simple climate model, several Earth System Models of Intermediate Complexity and a large number of Atmosphere-Ocean General Circulation Models (AOGCMs). b Year 2000 constant composition is derived from AOGCMs only. This is the IPCC report. This is about the most conservave predicon around

IPCC report Why is this hard?

Each scenario is fairly well deﬁned, so why is it so hard to make predicons for a given scenario?

Uncertainty in some parameters.

Feedback -- nonlinearity

Sheer complexity.

Can this be ﬁxed?

In principle, we can slow the rate of global warming by slowing the emission rates of heat-trapping gases—mainly carbon dioxide—and black carbon aerosol to the atmosphere. Some connued warming is inevitable. Stabilizing global temperature at its current level would be very diﬃcult because it would require cung the emission of heat- trapping gases all the way to zero. If and when zero emissions becomes possible, temperatures won't start to recover unl heat-trapping gases are actually removed from the atmosphere. Such removal happens naturally on me scales ranging from less than a year (e.g., black carbon aerosol) to many decades (e.g., carbon dioxide). Addionally, technical means exist to remove some heat-trapping gases (including carbon dioxide) from the atmosphere.

There is probably more economic and polical variables to consider than physics. (My opinion.)

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