London scoping - ukcip

Final Report
109
In this section we consider the implications of a changed climate upon the built environment.
We start by summarising the change in the urban winter and summer air temperatures arising
from changes in greenhouse gas emissions and the urban heat island effect. The bulk of the
section assesses the effects of these changes upon the heating and cooling requirements of
commercial buildings (the domestic building stock being dealt with in the next section). Much
of the analysis was done specifically for this project, and more details can be found in Wright
(2002). We also cover the potential impacts of a change in rainfall patterns.
The Changing Climate
In the future (Wright, 2002; Wilby [This document, Section 5]):
Warmer winters are expected in London, which will reduce heating requirements.
The number of heating degree days (roughly proportional to the space-heating
requirements in a well-heated building) will fall by between about 20 and 40%
depending on emissions scenario, by the 2080s.
Trend in the magnitude of the central London heat island: All seasons except
winter show the difference between rural and urban temperatures increases over the
21 st century. Over the heating season (October – April) it remains approximately
constant. This means that the difference between heating requirements around the
centre of London, and the suburbs, will not change.
Summers in London are expected to become warmer and drier. The heat island
effect also increases in summer. However, the regional rise in temperature is much
more important (of the order of 1 to 3°C) than the increase in heat island effect (of
the order of 0 to 0.5°C).
Summer evenings will be warmer. Each 1°C warming on summer nights equates
on average to more than an hour shift in the diurnal cycle; temperatures currently
experienced at 7pm would be experienced well after 8pm.
The frequency and intensity of summer hot spells will increase. The temperature
exceeded on average on one in 10 summer days will increase by between 4 and
7°C under climate change, depending on emissions scenario. This is
approximately double the increase in seasonal mean temperatures. Assuming the
annual extreme maximum temperature changes by a similar amount, from
Table 6.2, London’s summer extreme would be about 33.5ºC by the 2050s in the
two Medium Emissions scenarios, comparable with present-day Paris or Berlin. In
the 2080s this rises to 35ºC under the Medium-Low Emissions scenario, similar to
present-day Tokyo and New York. Note this is also hotter than all the present-day
European cities at a similar latitude.
However, since New York is about 11º latitude further south than London, it
experiences much stronger sunshine – sun strength on clear days is largely
determined by solar geometry and hence unaffected by climate change. Therefore,
although temperatures may be similar, the weather would never feel quite like
present-day New York. Cloud cover does decrease however in summer, by
between 8 and 17%, depending on scenario, by the 2080s so there will be more
sunshine on average. Note, however, that the level of confidence on changes in
cloudiness is given as ‘low’ in the UKCIP02 scenarios. This compares with high