As a dam built across a river causes a local deepening of the stream, so our atmosphere, thrown as a barrier across the terrestrial rays, produces a local heightening of the temperature at the Earth's surface. (Tyndall, 1862, quoted in Weart, 2004)
Gas (and formula) | Mixing ratio |
---|---|
major constituents | |
nitrogen (N2) | 0.78 |
oxygen (O2) | 0.21 |
argon (Ar) | 0.0093 |
trace gases | |
carbon dioxide (CO2) | 368 ppm |
methane (CH4) | 1745 ppb |
nitrous oxide (N2O) | 314 ppb |
ozone (O3) | 10-100 ppb |
(a) What proportion (as a percentage) of the Earth's planetary albedo is due to solar radiation reflected by the surface? Which regions of the world are likely to be mainly responsible for this contribution? (b) Calculate the difference between the rate of energy gain and the rate of loss for: (i) the Earth's surface; (ii) the atmosphere; and (iii) the whole Earth-atmosphere system (i.e. at the top of the atmosphere). What do you conclude about the Earth's GMST? (c) What proportion (as a percentage) of the longwave radiation emitted by the surface is absorbed by the atmosphere? (d) Translate the 114 units of longwave radiation emitted by the surface into a rate of energy transfer (in W m−2). Explain why your answer is consistent with the fact that the Earth's GMST is higher than its effective radiating temperature (−19 °C).
(a) The planetary albedo is the proportion of incoming solar radiation reflected or (scattered) directly back to space - 31 units according to Figure 12. Surface reflection contributes 9 units or (9/31) × 100% = 29%. Snow- or ice-covered surfaces (predominantly at high latitudes) are likely to be mainly responsible, given their high albedo. (b) (i) The total rate of energy gain by the Earth's surface is the sum of the appropriate downward-pointing arrows in Figure 2.12; i.e. (49 + 95) units = 144 units. The total loss rate is the sum of the upward-pointing arrows that originate at the Earth's surface: (30 + 114) units = 144 units. The difference is zero, so the surface is in a steady state; the GMST is not changing. (ii) Proceeding as in (i), the total rate of energy gain by the atmosphere is: (20 + 30 + 102) units = 152 units. The total rate of loss is: (95 + 57) units = 152 units. The difference is again zero. (iii) For the whole Earth-atmosphere system, the total rate of energy gain (solar radiation intercepted) is 100 units, and the total rate of loss is (31 + 57 + 12) units = 100 units, confirming that the whole system is also in a steady state. (c) The proportion is (102/114) × 100% = 89% (to 2 significant figures). (d) 100 units is equivalent to 342 W m−2, so 114 units is equivalent to (342 W m−2/100) × 114 = 390 W m−2. This is significantly higher than the rate of emission (236 W m−2; Section 1.2.1 ) from a body with an effective radiating temperature of −19°C. Since the rate of emission increases with increasing temperature, this implies that the Earth's GMST is higher than −19 °C.
Look back at Figure 12. What three factors could disturb the radiation balance at the top of the atmosphere?
Concentration | ||||
---|---|---|---|---|
Gas | Pre-industrial | 1998 | Atmospheric lifetime/years | Global Warming Potential |
natural greenhouse gases | ||||
CO2 | 280 ppm | 368 ppm | ∼100 | 1 |
CH4 | 700 ppb | 1745 ppb | 12 | 23 |
N2O | 270 ppb | 314 ppb | 114 | 296 |
synthetic halocarbons | ||||
CFC-11(CFCl3) | 0 | 268 ppt | 45 | 4600 |
CFC-12 (CF2Cl2) | 0 | 533 ppt | 100 | 101600 |
HCFC-22 (CHF2Cl) | 0 | 132 ppt | 12 | 1700 |
Gas | Radiative forcing/W m−2 | % Contribution |
---|---|---|
long-lived | ||
CO2 | 1.46 | 53 |
CH4 | 0.48 | 17 |
N2O | 0.15 | 5 |
halocarbons | 0.34 | 12 |
short-lived | ||
tropospheric O3 | 0.35 | 13 |
total | 2.78 | 100 |
The long-term hemispheric trend is best described as a modest and irregular cooling from AD 1000 to around 1850-1900, followed by an abrupt 20th century warming.
As a hybrid science-policy body, the IPCC must maintain credibility and trust vis-à-vis two rather different communities: the scientists who make up its primary membership, and the global climate policy community to which it provides input […] The IPCC's rules of procedure spell out a variety of methods designed to ensure its reports include the best available scientific knowledge and that they represent this knowledge fairly and accurately. Chief among these is the principle of peer review, traditionally one of the most important safeguards against bias and error in science.
if expert judgement varies too widely to provide a quasi-mechanical means of winnowing out bad science from good, why is peer review important? […] We maintain that peer review ought to be regarded as a [sometimes fallible] human process whose primary functions are to improve the quality of scientific work, to maintain accountability both inside and outside the scientific community, and to build a scientific community that shares core principles and beliefs even when it does not agree in detail.
The threat of a new ice age must now stand alongside nuclear war as a likely source of wholesale death and misery for mankind. (Nigel Calder, International Wildlife, July 1975)
This cooling has already killed hundreds of thousands of people. If it continues and no strong action is taken, it will cause world famine, world chaos and world war, and this could all come about before the year 2000. (Lowell Ponte, The Cooling, 1976)
Weather indicators | Observed changes* |
---|---|
hot days/heat index† | increased (likely) |
cold/frost days | decreased over most land areas during 20th century (very likely) |
continental precipitation | increased by 5-10% over 20th century in Northern Hemisphere (very likely), although it has decreased in some regions (e.g. N and W Africa and parts of Mediterranean) |
heavy precipitation events | increased at mid- and high northern latitudes (likely) |
frequency and severity of drought | increased summer drying and associated incidence of drought in a few areas (likely); in recent decades, frequency and intensity of droughts have increased in parts of Asia and Africa |
Physical indicators | Observed changes* |
---|---|
global-mean sea level | increased at average annual rate of 1-2 mm during 20th century |
duration of ice cover on rivers and lakes | in mid- and high latitudes of Northern Hemisphere, decreased by 2 weeks during 20th century (very likely); many lakes now freeze later in autumn and thaw earlier in spring than in 19th century |
Arctic sea-ice extent and thickness | thinned by 40% in recent decades in late summer (likely), and decreased in extent by 10-15% since 1950s in spring and summer |
non-polar glaciers | widespread retreat during 20th century |
snow cover | decreased in area by 10% since satellite observations began in 1960s (very likely) |
permafrost | thawed, warmed and degraded in parts of polar and sub-polar regions |
Low | Central estimate | High | |
---|---|---|---|
effects due to 20th century warming: | |||
thermal expansion | 0.3 | 0.5 | 0.7 |
glaciers | 0.2 | 0.3 | 0.4 |
Greenland ice sheet | 0.0 | 0.05 | 0.1 |
Antarctic ice sheet | −0.2 | −0.1 | 0.0 |
long-term ice-sheet adjustment | 0.0 | 0.25 | 0.5 |
total estimated | 0.3 | 1.0 | 1.7 |
observed | 1.0 | 1.5 | 2.0 |
We should recall that the IPCC was under considerable pressure in 1990 to make a statement attributing observed climate changes to human influence 'because if they don't, someone else will' (and indeed, did). The IPCC is a cautious body, and if the evidence is not available in the peer-reviewed literature to support a statement, it will not make it, no matter how great the interest in that statement might be. In the end, this caution resulted in the attribution statement made in the Second Assessment Report [in 1996] having much more impact than if it had been made prematurely.
In recent years, all major scientific bodies in the United States whose members' expertise bears directly on the matter have issued similar statements […] concluding that the evidence for human modification of climate is compelling.