Chap 2 : Is the climate changing?
Climate: the statistics of the atmosphere (chap 1).
Is the climate is presently changing?
Lengthy answer: long term trends and statistics
2.1. The temperature record
- most direct test of climate change
 oldest measurements
 best dataset
Problem: Temperature ≠ heat
Also temp varies over short times and regions
Long-term trends of interest
Example: two nearby temperature stations
Different offsets, but variations similar
Interested in CHANGES, not absolute temperatures
Measure relative to average - temperature anomaly
Monthly surface temperature anomaly in December 2009 (degrees Celcius).Reference
temperature is the average of the December temperatures from 2000 to 2009 at location
2.1.1 Surface temperature record
(a) Global annual average temperature anomalies (solid line = fit)
RHS axis shows averages
(b) Three different regions (reference temp = 1951-1980 average)
Results:
Clear, significant temp rise
2000-09: warmest on record
1906 -2005: average surface temp rose by 0.74 +/- 0.18 °C
Three distinct periods (1910-45: rise, 1945-76; plateau; 1976-: rise)
Rate of warming for century: 0.74°C
Rate of warming for past 50 years: 1.3°C/century (0.13 °C /decade)
Latitude/longitude trends; severest warming in the Arctic
Land warming greater than ocean (greater heat capacity of water)
Corrections for bias:
Site bias : (systematic error)
Urban sites (heat problems)
Rural sites (not many instruments)
Note that the Earth is mainly water, and even on land, mostly rural
Instrumental bias (random error)
Instrumental problems (bubble in liquid-in-glass thermometer)
Change in instruments, observing practices, station locations
Errors caused by transcription or digitization
These add noise - occur no more frequently in one direction than in the other
Note: noise makes short time-scale trends misleading
Berkeley study: robust results worldwide
2.1.2. Satellite temperature record
Measurements from orbit (since 1978)
Averaged over the lower troposphere (up to 8 km)
Constructed from 12 satellites
Satellite measurements of global monthly average temperature of atmosphere layer
between 0 and 8 km from surface
Measurements since the late 1970s; merging issues with inter-calibration
Diurnal correction; stratospheric cooling
Result: 0.13 °C per decade; agrees with surface results
2.1.3. Ocean temperature record
Temp measurement of bulk ocean (depth of 1-4 km)
Ocean temperature anomaly (to depth 700m), measured relative to 1957-1990 average
Rising over the past few decades.
Small rise (large heat capacity of water)
2.2 Ice melt measurements
Ice melts at 0°C
Can rising temperatures be seen by examining ice?
2.2.1 Ice melt (land): glaciers and ice sheets
Glaciers form when snow that falls during the winter does not melt
completely during the summer. Snow accumulates, then compresses under
its own weight to form giant ice glaciers.
Akin to ultra-slow river. Glacier flows slowly down mountain, with melting
at the bottom and snow falling on top
Change in average glacier length from 1950
Ice sheets: huge glaciers in Greenland and Antarctica
3000 m thick, Millions of square km
Reservoir of world's freshwater (complete melt = sea rise of 100m)
Satellite measurement of Greenland ice sheet in billions of tons 2002-2009
Ice sheet loss: 200 billions tons/yr (Greenland)
Comparable ice-sheet loss in Antarctica
2.2.2 Ice melt (sea)
Sea ice forms when cold air blows over water. Only a few meters thick
because ice is a good insulator (igloo)
Extent and thickness of sea-ice have both retreated
Arctic sea–ice area: % deviation from 1979-2000 mean each September
2.3 Sea-level measurements
Temperature change should be detected in sea level rise because:
(a) Melting of grounded ice (less than half)
(b) Thermal expansion (about half)
(c) Changes in amount of water stored on land
Results
Global annual average sea level anomaly , relative to 1961-1990 average
Sea level has risen by 15 cm over full century (1.5 mm/yr )
Past 40 years: 1.8 mm/yr
Past 10 years: 3.1 mm/yr
Summary
Consistency of results:
1. Surface and satellite temperature datasets suggest same degree of
temperature rise
2. Melting of sea and land ice suggest warming
3. Sea-level measurements suggest warming
independent lines of evidence
Different datasets prone to different uncertainties
No single error could affect all data sets
Consistent result
Conclusions:
1) global climate has warmed about 0.7°C over the last century
2) warming is accelerating