Chapter 15: Recent and Future Climate Part 1—The present and recent past

Climate over the past 150,000 years The Earth System, ed. 3, Fig These estimates come from oxygen isotopes in deep-sea carbonate cores, as we discussed in Ch. 14 Interglacials

More recent climate variations More recent, land-based records come from pollen and from historical accounts (many of which are biased towards European climates) The Earth System, ed. 3, Fig Pollen data Historical records Holocene Little Ice Age

Alpine glaciers are retreating The Argentiere glacier in the French Alps is retreating, as are 90% of the glaciers in this region The Earth System, ed. 3, Fig. 15-2

What caused the Little Ice Age? We are still uncertain about this, but one possibility is an increase in volcanic activity Volcanoes like Mt. Pinatubo (at right) inject millions of tons of SO 2 into the atmosphere Oxidation of this SO 2 produces sulfate aerosols, which cool Earth’s climate by increasing its albedo MountainWatch_Bishkek/presspack/photos.htm

The “Year without a summer” (1816) Image from Wikipedia The 1815 eruption of Mt. Tambora in Indonesia is thought to have caused this unusually cold year

Solar variability and climate The Earth System, ed. 3, Fig Another possible cause for the Little Ice Age is solar variability, especially if solar luminosity was lower during the Maunder minimum in sunspot activity However, variations in solar luminosity on the time scale of the sunspot cycle are small (0.1%), so it is not clear that this can work

More recent climate records from thermometers show that the climate has been warming rapidly for at least the last 40 years

2013 IPCC Report, Technical Summary Trends in different variables showing evidence for climate change Sea level has increased by ~25 cm October arctic sea ice extent has decreased by almost 50%

Projections of future sea level rise The total sea level rise by the year 2100 is projected to be between 0.5 m and 1.2 m This is enough to nearly submerge some small island nations –This projection does not include possible melting of polar glaciers (Antarc- tica and Greenland) 2013 IPCC Report Technical Summary

The Maldives World’s lowest country –Average elevation: 1.5 m Population: 328,000 They are very concerned about sea level rise!

How do we know, though, that the changes in surface temperature are caused by human- induced (anthropogenic) global warming? –After all, climate has varied over a variety of time scales, particularly the glacial-to-interglacial time scale, for a variety of natural reasons To find the answer to this question, we compare climate models with observations The IPCC (Intergovernmental Panel on Climate Change) compares the results of many (roughly 15-20) different climate models and produces consensus predictions

Global climate models Image from Wikipedia These global climate models, or GCMs, incorporate all the physical and dynamical processes that we discussed in Chapters 3-5

Climate projections with and without anthropogenic forcing WithoutWith This is the key figure from the 2013 IPCC report Result: Only by including anthropogenic forcing (i.e., greenhouse gas emissions) can the models reproduce the observed global warming 2013 IPCC Technical Summary, Fig. TS.9

Which greenhouse gases are most important? Greenhouse gases we have studied: –CO 2 –H 2 O (feedback only) –CH 4 (methane) –N 2 O (nitrous oxide) –Freons and freon substitutes

Relative contributions of different greenhouse gases to warming IPCC 2013 report: Technical Summary By comparison, one CO 2 doubling  4 W/m 2 of forcing Bottom line: CO 2 is by far the most important anthropogenic g.g.

So, the biggest problem is CO 2 from fossil fuel burning. To see why, let’s look at CO 2 fluxes…

Rising CO 2 due to Fossil Fuel Combustion Volcanism0.06 Gt(C)/yr Organic C weathering0.05 Carbonate precipitation0.17 Respiration & decay60

Rising CO 2 due to Fossil Fuel Combustion Volcanism0.06 Gt(C)/yr Organic C weathering0.05 Carbonate precipitation0.17 Respiration & decay60 Fossil Fuels:6 Gt(C)/yr(1990) 9.2 Gt(C)/yr(2010) * 9.7 GT(C)/yr (2013) * The fossil fuel CO 2 source is big compared to the natural volcanic source, but small compared to respiration and decay What does this mean? * CDIAC (Carbon Dioxide Information Analysis Center, Oak Ridge, TN

Actual vs. projected CO 2 emissions Raupach et al., PNAS (2007) Actual emissions up through 2005 equal or exceed the highest projections from the 2001 IPCC report 2010: 9.2 GtC/yr (CDIAC) 2013: 9.7 GtC/yr

What does this mean? The fact that fossil fuel emissions are large compared to volcanic input means that humans are introducing a large perturbation into the natural carbon cycle The fact that fossil fuel emissions are small (~10%) relative to fluxes in the terrestrial organic carbon cycle means that imbalances in the organic carbon cycle (e.g., deforestation, reforestation) can have a substantial effect on atmospheric CO 2 levels

Which fossil fuels are being burned? Source: Carbon Dioxide Information Analysis Center (CDIAC) See also The Earth System, ed. 2, Fig (older figure) Coal Oil Natural gas

Burning of Fossil Fuels: ReservoirBurn rate * Lifetime (yrs) Coal3500 GtC4.1 GtC/yr 850 Oil Gas 500 ** 1.8 ** 280 Cement0.5 Total:4670GtC 9.7 GtC/yr 480 *Lifetime: t = R/F **2013 values Values from:

Relative efficiency of fossil fuels with respect to CO 2 production lbs CO 2 /MWhr 10 4 MWhr/lbsCO 2 Relative efficiency Coal Oil Gas Source: So, burning gas to produce electricity is much more efficient than burning coal, in terms of the amount of CO 2 released Are the new finds of shale gas good from an environmental standpoint?

What are the fossil fuels being used for in the U.S.?

The rate of burning is increasing. Rate of increase: 2.0 ppm/year or 4 Gt(C)/yr This means that, at present, about half of the CO 2 emitted from fossil fuel burning stays in the atmosphere, while the other half is taken up by the Earth system 

Fraction of emitted CO 2 retained in different reservoirs The fraction of CO 2 retained by the atmosphere (the airborne fraction) varies from year to year –Strongly affected by the El Niño cycle That said, there appears to be a long-term increase, perhaps because we are gradually using up the buffering capacity of the surface ocean and making it more acidic: CO 2 + CO 3 = + H 2 O  2 HCO 3  Canadell et al., PNAS (2007) AF

Where do these CO 2 emissions come from, geographically speaking?

2004 burning rates by geographic region Consumption rate*, Region Gtons(C)/yr % of total North America Central and South America0.4 6 Western Europe Eastern Europe (inc. Russia) Middle East0.4 6 Africa0.3 5 Oceania (Australia and Japan)0.4 6 China and Vietnam TOTAL6.4

2008 CO 2 emission rates RankCountry% total emissions Per capita emissions * 1China United States European Union Russia India Japan * Millions of metric tons CO 2 per person Source: Union of Concerned Scientists

2013 CO 2 emission rates RankCountryEmissions Gt(C)/yr % total emissions 1China United States EU India0.66 5Russia0.55 6Japan0.44 WORLD Source: Wikipedia

CO 2 emissions will grow as countries industrialize Mtons(CO 2 )/yr

The big issue (for international climate negotiations) Should developing countries, like China and India, who have big total CO 2 emissions, but small per capita emissions, be required to make the same future cuts in emissions required of industrialized nations?