Global Warming

Comparison withobservations 105

Our ability to quantify the human influence on global climate is currently

limited because the expected signal is still emerging from the noise of

natural climate variability, and because there are uncertainties in key

factors.These includethe magnitude andpatterns oflong term natural

variability and the time-evolving pattern of forcing by, and response to,

changes in the concentrations of greenhouse gases and aerosols, and land

surface changes. Nevertheless, the balance of evidence suggests a

discernible influence on global climate.

Since 1995 a large number of studies have addressed the problems of
detectionandattribution^18 of climate change. Better estimates of natural
variability have been made, especially using models, and the conclusion
reached that the warming over the last one hundred years is very un-
likely to be due to natural variability alone.^19 In addition to studies using
globally averaged parameters, there have been detailed statistical studies
using pattern correlations based on optimum detection techniques ap-
plied to both model results and observations. For example, Figure 5.23
shows a comparison between simulated and observed estimates of zonal
mean temperature change as a function of altitude. Taking these studies

o

Latitude Latitude

Pressure (hPa)

-1.2 -0.8 -0.4 0 0.4

(a)

0.4

0.4

0.2

0.0

0.0

0.4

0.4

0.2

0.4 0.0

0.2

0.0 0.2 0.2

90 oN 60 oN 30 oN 0 30 oS 60 oS 90 oS

800

500

300

200

100

Pressure (hPa)

-1.2 -0.8 -0.4 0 0.4

(b)

0.4

0.2

0.0

0.4

0.2

0.2 0.2

0.0

0.4

0.0

0.6

90 oN 60 oN 30 oN 0 30 oS 60 oS 90 S

GSO

0.4

800

500

300

200

100

Observations

-0.2 -0.2

-1.0

-1.3

-2.2

-0.2

-0.2

-0.4

-0.9 -1.2

Figure 5.23Simulated and observed zonal mean temperature change as a
function of latitude and height. The changes plotted are the differences between
the 1986–1995 decadal average and the twenty-year average from 1961 to

1980. The contour interval is 0.1◦C. (a) simulated changes taking into account
      increases in carbon dioxide, sulphate aerosols and the effect of observed
      changes in stratospheric ozone; (b) observed changes. A common pattern of
      stratospheric cooling and tropospheric warming is evident in the observations
      and both model experiments; the stratospheric cooling is partially due to the
      increase of carbon dioxide (see also Figure 4.2(b)) and the reduction in ozone
      (see Chapter 3).