Climate Change ● The 4-point consensus view of global warming ● Some objections impartially considered ● Evaluating Impacts Nb: this is my personal view, and I am speaking in a personal capacity. This talk is available from...

(1) Global Warming is happening...

(2) We're causing it...

(2)...we're causing it, continued

(3) It will get “worse”

(4) This will be a Bad Thing ● Sea level rise is bad, but slow ● Temperature rise regionally varying – winners and losers, political tradeoffs ● Ecological impacts – important, but I don't know

Common myths, impartially considered ● The “urban heat island” - negligible effect ● Satellite temperatures - show warming too ● The “hockey stick” controversy ● “The Day After Tomorrow” - will not happen ● Little relation between the Ozone hole and GW ● CO2 increase is anthropogenic (more than...)

Satellite temperature measurements ● Mears et al: °C/decade ● Spencer and Christy version 5.1, °C/decade ● Fu et al, 0.2 °C/decade (May 04) ● Vinnikov and Grody, with 0.22°C to 0.26°C per decade (Oct. 03) ● Surface record: 0.06 °C/decade over the past century and 0.15 °C/decade since

Satellites (2)

Urban Heat Island (1) ● Cities tend to be hotter than the surrounding countryside. But (for the purposes of the temperature record) we care about trends – ie are cities getting even warmer? ● Hansen et al. (JGR, 2001) adjusted trends in urban stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record. Of these adjustments, 42% warmed the urban trends: which is to say that in 42% of cases, the cities were getting cooler relative to their surroundings rather than warmer. One reason is that urban areas are heterogeneous, and weather stations are often sited in "cool islands" - parks, for example - within urban areas. ● IPCC: UHI does not exceed about 0.05°C over the period 1900 to 1990, because: – land, sea, and borehole records are in agreement – the trends in urban stations for 1951 to 1989 (0.10°C/decade) are not greatly more than those for all land stations (0.09°C/decade). – the differences in trend between rural and all stations are also virtually unaffected by elimination of areas of largest temperature change, like Siberia, because such areas are well represented in both sets of stations.

Urban Heat Island (2) ● Peterson, J Clim, 2003: "Assessment of urban versus rural in situ surface temperatures in the contiguous United States: No difference found" indicates that the effects of the UHI may have been overstated, finding that Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures. This was done by using satellite-based night-light detection of urban areas, and more thorough homogenisation of the time series (with corrections, for example, for the tendency of surrounding rural stations to be slightly higher, and thus cooler, than urban areas). ● Parker, Nature 2004: attempts to test the urban heat island theory, by comparing tempature readings taken on calm nights with those taken on windy nights. If the urban heat island theory is correct then instruments should have recorded a bigger temperature rise for calm nights than for windy ones, because wind blows excess heat away from cities and away from the measuring instruments. There was no difference between the calm and windy nights, and the author says: we show that, globally, temperatures over land have risen as much on windy nights as on calm nights, indicating that the observed overall warming is not a consequence of urban development. ● Publication bias: Peterson notes that essentially all large-scale studies showed some urban centres cooling. But no individual city studies did. People were reporting what they expected to see.

“Hockey Stick” controversy... or, The temperature over the last 1-2 kyr ● (dark blue): P.D. Jones, K.R. Briffa, T.P. Barnett, and S.F.B. Tett, 1998, The Holocene 8, 1998, ● (blue): Michael E. Mann, Raymond S. Bradley, and Malcolm K. Hughes, 1999, Geophysical Research Letters Vol. 26, No. 6, p. 759, March 15, ● (light blue): Crowley and Lowery 2000 (Ambio 29, 51), Science v289 p.270, 14 July ● (lightest blue, somewhat green): Briffa et al. (2001) J Geophys Res 106, ● (light green): Esper, J., E.R. Cook, and F.H. Schweingruber, 2002, Science ● (yellow): Mann, M.E. and P.D. Jones, 2003 Geophysical Research Letters, August ● (orange): Jones, P.D. and M.E. Mann Reviews of Geophysics 42, RG2002, 6 May ● (red-orange): Huang, S Geophys. Res, Lett. 31, L13205, doi: /2004GL ● (red): Moberg, Anders et al., 2005, Nature

The “day after tomorrow”... will not happen ● Simulations using the HadCM3 climate model of the Atlantic thermohaline circulation from (using historical variations of greenhouse gases, sulphate aerosol, solar radiation and volcanic dust). The simulations show a freshening of the Labrador Sea from , as has been seen in observations, but this is associated with a slight strengthening of the thermohaline circulation over the same period, rather than a weakening as has sometimes been suggested. When the simulations are extended forward from (using a projection of future greenhouse gases and aerosols), both trends are reversed, with a salting in the Labrador Sea and a weakening thermohaline circulation

Ozone hole/depletion and climate change ● Although they are often interlinked in the popular press, the connection between global warming and ozone depletion is not strong. ● Global warming from CO2 radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes. ● Conversely, ozone depletion represents a radiative forcing of the climate system. O3 losses over the past two decades have tended to cool the surface. ● One of the strongest predictions of the GW theory is that the stratosphere should cool. However, although this is observed, it is difficult to use it for attribution (for example, warming induced by increased solar radiation would not have this upper cooling effect) because similar cooling is caused by ozone depletion.

Other stuff... ● CO2 rise is anthropogenic ● Global dimming ● Temperature rise non-uniform: largest over NH land in winter

Evaluating Impacts ● Not really my thing: just some ideas ● Of the Consensus View, the weakest point is “...and it will be a bad thing”. Which is not to say its wrong, just harder to evaluate. Risk: change from what we (and ecologies) are adapted to. ● Sea Level Rise is fairly obviously bad, but probably slow (surprises?) ● Ecological impacts I don't know: but difficult political choices: how many flights to go skiing are people prepared to forgo, in order to save polar bears (were that the choice).