The heavier temperature lines 160,000 BP to present reflect more data points for this time period, not necessarily greater temperature variability.

Transcription

1 Climate change Issues for Science A-30 students to consider Is climate getting warmer? How much warmer (if yes)? How do we know? If climate is getting warmer, why is it getting warmer? How do we know? Should we be worried? Should we sacrifice our (other peoples?) economic well-being to reduce emissions of CO 2? How do we decide? What do we study to help us decide?

2 The heavier temperature lines 160,000 BP to present reflect more data points for this time period, not necessarily greater temperature variability. Climate and Atmospheric History of the past 420,000 years from the Vostok Ice Core, Antarctica, by Petit J.R., Jouzel J., Raynaud D., Barkov N.I., Barnola J.M., Basile I., Bender M., Chappellaz J., Davis J. Delaygue G., Delmotte M. Kotlyakov V.M., Legrand M., Lipenkov V.M., Lorius C., Pépin L., Ritz C., Saltzman E., Stievenard M., Nature, 3 June 1999.

3 Millenial NH temperature trend [IPCC, 2001]

4 Global q Temperature y Changeg 1000 yrs (after T. Crowley, Science 289, 271, 2000) Model vs data greenhouse gases solar activity volcanoes pollution aerosols Model Model Temperature Anomaly (C) Temperature Anomaly (C) o C Crowley Mann YEAR i i

5 Measurements of the ocean temperature have been aggregated to show that the whole ocean has warmed, by about 0.2 C. Is this a definitive test of Global Warming, or of some of the elements of the concept?

6 Things we learned about in Science A-30, and then read about in the Kolbert article from the New Yorker: 1. Stefan-Boltzmann Law and effective Temperature of the Earth. 2. Greenhouse effect, role of water vapor, clouds, and CO Keeling CO 2 record, Vostok ice core data for past CO Feedbacks on climate involving water vapor, ice-albedo, and CO Ocean uptake of CO 2 and acidification. 6. Ocean uptake of heat. 7. Ocean temperatures and hurricanes (final lectures of Science A-30). 8. Uptake of CO 2 by land plants, land use legacies. 9. Low-pass filter in the climate system (bucket demo).

7 The Greenhouse Effect: influence of atmospheric absorption and emission of planetary (infrared) radiation incoming solar radiation (F s ) (visible, near infrared) σt e 4 σt e 4 far infrared radiation from the atmosphere z=h reflected solar (A) σt g 4 terrestrial (far infrared) radiation from the surface

8 Feedback is taking place in the climate system because the number of absorbing "layers", n (also called the "optical thickness") can depend on the temperature of the atmosphere, or on other climatic parameters such as precipitation. The effective temperature Te depends on the albedo might also which might also depend on the climate itself. Thus we allow for n and A to be functions of Tg : Tg = { [n(tg) + 1][1 - A(Tg) ] Fs/(4σ) } 1/4 Tg depends on itself! This self-interaction is called feedback, and it may involve complexity, and it may manifest itself over long or short time scales.

9 FEEDBACKS INVOLVING ABSORPTION OF IR (HEAT) Examine some of the most important feedbacks in the Earth s atmosphere. water vapor feedback. Temperature increases atmosphere H 2 O increases (Clapeyron equation) atmospheric absorption increases (n) Temperature increases + This is the strongest feedback mechanism in the atmosphere. It is also the best understood since it is based simply on the measured increase in water vapor pressure increase with temperature (Clapeyron equation). cloud feedback terrestrial radiation Temperature increases atmosphere H 2 O increases (Clapeyron equation) cloudiness increases (n) Temperature increases + This is a very strong feedback that is not well understood because it is hard to know whether or how much cloudiness would increase as temperature does cloudiness depends on upward air motion more than on T or H 2 O directly.

10 FEEDBACKS INVOLVING ALBEDO (continued) ice-albedo feedback solar radiation Temperature increases polar ice recedes Albedo decreases + Temperature increases This is a very strong feedback when there is a lot of polar ice, for example, at the height of the last ice age. It works both ways, helping the ice sheets to advance as the earth cooled, by amplifying the cooling, and accelerating the retreat of the ice sheets as the climate started to warm. There is rather little polar ice today, so this feedback is not likely to play a major role in climate change. snow albedo feedback (class discussion) snow surface-t feedback (class discussion)

11 Climate model results from the IPCC Atmospheric change T (C) Double CO Double CO 2 + water vapor feedback +1.9 Double CO 2 + water vapor feedback + clouds + ice-albedo feedback (median 2.5) The "climate sensitivity factor" is strongly affected by feedbacks. Model-computed changes in global mean temperatures due to specified changes in atmospheric composition and radiative properties.

12 PROJECTIONS OF FUTURE CO 2 CONCENTRATIONS [IPCC, 2001]

13 FUTURE TEMPERATURE PROJECTIONS FROM CLIMATE MODELS (IPCC, 2001)

14 PROJECTIONS OF FUTURE CO 2 CONCENTRATIONS [IPCC, 2001] CO 2 emissions (PgC/yr) CO 2 concentration (ppm)

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16 Temperature Change ( o C) Global Temperature Climate Model Mt. Pinatubo eruption Effect of a major volcanic eruption on climate ( after Hansen et al., 1993) A test of climate models.

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18 cloud feedback solar radiation Temperature increases atmosphere H 2 O increases (Clapeyron equation) cloudiness increases (n) Albedo increases Temperature decreases This is also very strong feedback that is not well understood because it is hard to know whether or how much cloudiness would increase as temperature does, and because of the trade-off (competition) between the effects of clouds on absorption of infrared radiation versus reflection of solar radiation. Low-altitude clouds affect albedo more than they affect ir radiation, and conversely for high clouds vegetation feedback solar radiation Temperature increases deserts expand Albedo increases - - FEEDBACKS INVOLVING ALBEDO Temperature decreases This is a very complex feedback that will take a long time to be realized. Maybe deserts won't expand, or plants will be greener because there is more CO 2?

19 Climate CO 2 negative feedback positive

20 Does it make sense to think of a global mean temperature, and to focus our attention on that number when we examine climate change?

21 Future climate change is likely to be most dramatic at high latitudes

22 [base yrs: ] But wasn't the weather unusually cold in ? Not over the globe.

23 includes Pinatubo, ENSO Temperature changes over the globe since 1946, estimated by fitting a straight line to annually averaged data from surface temperature stations (nonadjusted for station effects). Note the influence of the dates used on the T changes derived from data [Pinatubo, ENSO, disappearing stations]. excludes Pinatubo, ENSO (from the graphics engine at data/update/gistemp/maps/)

24 Correlation: { T, soil moisture index} CCSM1-Carbon Control Simulation DJF JJA Positive correlation warmer-wetter; or cooler-drier Negative correlation warmer-drier; or cooler-wetter slide courtesy Inez Fung [I. Fung, S. Doney, et al.]]

25 Mean temperature, C Thompson Flin Flon Lynn Lake Norway House Gillam NOBS Low T, low ppt high T, high ppt Precipitation anomaly, cm Recent climate variations in central Canada have been [cold:dry] and [warm:wet] what happened?

26 g C m -2 yr CO 2 uptake (NEE) Uptake emission 2 1 C T anomaly (warming) 5 cm 0-5 Wetness (wetter, ) Boreal Forest: Response of CO 2 uptake to temperature anomaly and to 3-year lagged climate moisture index anomaly (10 years of Harvard data from Manitoba) Water balance & (temperature) explain the transition: source sink

27 Holdridge Life Zones & potential vegetation: Mean T, Precip, and E/P control vegetation cover: warmer-drier leads to strong degradation in the tropics. 125 E/P P Data courtesy of D. Skole 1.5 T Holdridge life zones (Holdridge 1967)

28 Climate forcing due to changes in concentrations of greenhouse gases, atmospheric aerosols, and clouds, since 1850 (Hansen, 2001).

29 Hansen s predictions, and Wofsy s lucky shirt In 1981, Hansen predicted that carbon dioxide warming should emerge from the noise of natural climate variability around the year In 1988, he made a famous prediction about future climate trends, for which he is taken to task by critics (next slide). In 1992, Wofsy was working on a project where an aircraft was attempting to take samples of the Arctic winter stratosphere. Weather on the ground caused the cancellation of many flights. He started wearing his lucky shirt, and showed that the plane had a much higher chance to make a flight when he was wearing the shirt. How do we assess the credibility of these various predictions?

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31 Summary Some aspects of the climate debate have shifted today since the time of Hansen's testimony (and predictions of warm years to come) in 1988 and the IPCC 1995 report. The changes in recent decades, and in the last 100 years, may place current temperatures outside the range for the last several centuries, although not outside the range for the last 10,000 years (see sequence of temperature reconstructions on the Science A-30 web site). Hansen's prediction of record warmth in the 1990's was correct. The existence of a warming trend is more widely accepted, and the debate has shifted to why climate has warmed (human-caused, or natural) and to the effects of climate change (damaging, neutral, or beneficial). Analysis of the geological record shows that the past has witnessed very dramatic climate shifts in very short time intervals, sometimes leading to major extinctions of ancient organisms. Should we worry about abrupt climate change?