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

Audience Test How much has global temperature risen over the past 100 years? How much might sea level rise over the next 100 years?

(1) Global Warming is happening...

GW is happening (cont...)

(2) We're causing it

(2) We're causing it (cont...)

(2)...we're causing it (...cont #2)

(3) It will get “worse”

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

Common myths, impartially considered ● Satellite temperatures - show warming too ● The “urban heat island” - negligible effect ● 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...) ● Hurricanes (and severe weather)

Satellite temperature measurements ● Mears et al: °C/decade ● Spencer and Christy version 5.2, °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

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 Osbourn and Briffa:

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.

CO2 emissions... reductions? ● Of the "frontrunners" one is an order of magnitude bigger than the rest: Extend UK participation in EU carbon trading scheme (4.2). Means: don't actually produce less CO2, but buy permits to emit it. ● Of the "emerging" category, the two biggest are Introduce ways to store carbon pollution underground ( ) (i.e., don't produce any less, just...) and Force energy suppliers to use more offshore wind turbines (Up to 1). Which would actually save CO2. ● In the "difficult" category the biggest is Change (read: enforce) road speed limits (1.7) - a surprisingly large number. The bill will set out a statutory commitment to cut CO2 emissions by 60% from 1990 levels by 2050, requiring annual cuts way above anything the Labour government has achieved so far... [Guardian, 2006/11/15]

Hurricanes and severe weather ● There is some evidence for hurricanes becoming more stronger; and some suggestions for this in the future in the models ● But most impacts come from more people living near the beach ● Katrina was unlucky

Stern Review ● Received to rapturous applause from UK politicians and press ● Apparently intended to be used to beat Bush over the head ● Not much science (one chapter of 27). Purports to take the IPCC position ● Finds higher costs of climate change, and lower costs of fixing this, than just about everyone else, but doesn't really explain why ● Objections raised in the blogosphere and beyond, mostly to the economics ● Nordhaus: Stern is using a social discount rate that is essentially zero. And The Review's unambiguous conclusions about the need for extreme immediate action will not survive the substitution of discounting assumptions that are consistent with today's market place. ● The review uses a high-end emission scenario (A2) together with what appears to be excess weight to higher climate sensitivities: Several new studies suggest up to a 20% chance that warming could be greater than 5°C. I think sensitivities that high are distinctly unlikely. ● Won't work.

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).