Can we defuse the global warming time bomb? ( Page: Previous Next )
Global average surface temperature has increased about 3/4 °C (1.35 °F) during the period of extensive instrumental measurements, i.e., since the late 1800s. Most of the warming, about 1/2 °C (0.9 °F), occurred after 1950. The causes of observed warming can be investigated best for the past 50 years, because most climate forcings were observed then, especially since satellite measurements of the sun, stratospheric aerosols and ozone began in the 1970s. Furthermore, 70% of the anthropogenic increase in greenhouse gases occurred after 1950.
Figure 4. Climate forcing over the past 50 years due to six mechanisms (GHC = long-lived greenhouse gas). The tropospheric aerosol forcing is highly uncertain (Reference 1b).
These forcings have been used to drive climate simulations for 19511998 with the NASA Goddard Institute for Space Studies SI2000 climate model (Reference 1b). This model has a sensitivity of 3/4 °C per W/m2, consistent with paleoclimate data and typical of other climate models. The largest suspected flaws in the simulations are the omission of poorly understood aerosol effects on cloud drops and a probable underestimate of black carbon changes. The first of these is a negative forcing and the second is positive, so these flaws should be partially compensating in their effect on global temperature.
Figure 5. Simulated and observed global temperature change for 19512000, and simulated planetary energy imbalance (Reference 1b).
The most important quantity is the planetary energy imbalance (Figure 5d). This imbalance is a consequence of the long time that it takes the ocean to warm. We conclude that the Earth is now out of balance by about 0.5 to 1 W/m2, i.e., there is that much more solar radiation being absorbed by Earth than heat being emitted to space. One implication of this imbalance is that, even if atmospheric composition does not change further, the Earth's surface will eventually warm another 0.40.7 °C.
Box 4. Planetary heat storage: Ice, air, land and ocean.
Estimates of the energy used to melt ice and warm the air, land and ocean in the past 50 years.1
Ice melting: assume that the 10 cm rise in sea level between 1950 and 2000 was due to melting ice (thermal expansion of warming ocean water contributes about half of the rise, but this error is partly balanced by melting sea ice and ice shelves, which do not raise the sea level). If the initial temperature of the melted ice was 10 °C and its final temperature was that of the mean ocean surface (+15 °C), then the energy used is 105 cal/g (80 cal/g for melting). The heat storage is thus 10 g/cm2 × 105 cal/g × 4.19 J/cal × surface area of Earth (~5.1 × 1018 cm2) × ocean fraction of Earth (~0.71) ≈ 1.6 × 1022 J ≈ 1 watt-year.
Air warming: for a 0.5 °C increase in air temperature, the heat storage in the air is: 0.5 °C × the atmospheric mass of air (≈ mass of 10 m column of water ≈ 1000 g/cm2) × heat capacity air (≈ 0.24 cal/(g·°C) × 4.19 J/cal × surface area of Earth ≈ 0.26 × 1022 J ≈ 0.16 watt-year.
Land warming: The mean depth of penetration of a thermal wave into the Earth's crust in 50 years, weighted by ΔT, is about 20 m. If the Earth's crust has a density of ~3 g/cm3 and a heat capacity of ~0.2 cal/(g·°C), and the fractional land coverage of Earth is about 0.29, then the land heat storage is 2 × 103 cm × 3 g/cm3 × 0.2 cal/(g·°C) × 0.5 °C × 4.19 J/cal × surface area of Earth × 0.29 ≈ 0.37 × 1022 J ≈ 0.23 watt-year.
Ocean warming: Levitus (Reference 4) found a mean ocean warming of 0.035 °C in the upper 3 km of the ocean. The heat storage is thus: 0.035 °C × 3 × 105 g/cm2 × 1 cal/g × 4.19 J/cal × surface area of Earth × 0.71 ≈ 16 × 1022 J ≈ 10 watt-years.
1 Note that 1 J = 1 W·s, the number of seconds in a year ≈ π × 107, and the surface area of the Earth ≈ 5.1 × 1018 cm2; therefore, 1 watt-year over the entire surface of the Earth ≈ 1.61 × 1022 J.
The time bomb
The goal of the United Nations Framework Convention on Climate Change, produced in Rio de Janeiro in 1992, is to stabilize atmospheric composition to "prevent dangerous anthropogenic interference with the climate system" and to achieve this in ways that do not disrupt the global economy. The United States was the first developed country to sign the convention, which has since been ratified by practically all countries. Defining the level of warming that constitutes "dangerous anthropogenic interference" (DAI) is thus a crucial but difficult part of the global warming problem.
Figure 6. Surface melt on the Greenland ice sheet descending into a moulin. The moulin is a nearly vertical shaft worn in the glacier by surface water, which carries the water to the base of the ice sheet. (Photo courtesy of Roger Braithwaite and Jay Zwally.)
This qualitative picture of nonlinear processes and feedbacks is supported by the asymmetric nature of glacial cycles (Figure 3) and the high rate of sea level rise associated with rapid warming. Although building of glaciers is slow, limited by annual snowfall rates, once an ice sheet begins to collapse its demise can be spectacularly rapid.