Environment Counts | Sea level and energy budgets 1961 to 2008
Author: Geoff Zeiss – Published At: 2012-03-07 12:30 – (1104 Reads)
In the last two Intergovernmental Panel on Climate Change (IPCC) assessments, the sum of observed contributions to seaâ€level rise has consistently been less than the observed rise. This international study with authors from Australia, the U.S., Canada, the U.K. and the Netherlands considers energy and sea level budgets together for the past five decades from 1972 to the present and concludes that the sum of the observed contributions agrees with the observed rise within the error bars. It infers that since the 1990s anthropogenic aerosols and moderate volcanic activity have contributed significantly to the Earth’s energy balance. GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L18601, 8 PP., 2011
Sea level budget
In the last two Intergovernmental Panel on Climate Change (IPCC) assessments, the sum of observed contributions to seaâ€level rise has consistently been less than the observed rise. Closing the seaâ€level budget requires accurate estimates of ocean thermal expansion and the related ocean heatâ€content change, by far the largest storage of heat in the climate system. This study considers energy and sea level budget together for the past five decades from 1972 to the present.
This study also includes a new estimate of groundwater depletion, which were lacking in previous studies of seaâ€level change.
The figure shows the global seaâ€level budget from 1961 to 2008. (a) The observed sea level using coastal and island tide gauges (solid black line with grey shading indicating the estimated uncertainty) and using TOPEX/Poseidon/Jasonâ€1&2 satellite altimeter data (dashed black line). The two estimates have been matched at the start of the altimeter record in 1993. Also shown are the various components as described in the text. (b) The observed sea level and the sum of components. The estimated uncertainties are indicated by the shading. The two time series are plotted such that they have the same average over 1972 to 2008.
The sum of the contributions to sea level rise agrees within the error bars with the observed rate of sea-level rise. Thermal expansion contributes about 40% of the observed rise since 1972. The sum of the glacier and thermosteric contributions explains approximately 75% of the observed rise since 1972. Both the Greenland and Antarctic ice sheets are an important component in closing the sea-level budget, particularly since 1993.
The larger differences between the sum of contributions and the observed sea-level change occur in the 1960s when the data coverage was poorer.
The energy budget calculations imply that the rapidly increasing surface temperatures from the mid 1970s to the mid 1990s are consistent with increasing greenhouse gas concentrations and the steady to slightly weaker negative forcing (reflectance of sun radiation) from anthropogenic aerosols. A subsequent increase in the inferred negative aerosol (or some unidentified) forcing in the 2000s is required for energy balance, as there was little surface warming over the last decade even though greenhouse gas concentrations continued to increase (with a small decrease in solar input) and the ocean continued to warm and sea level continued to rise.
The figure shows the Earthâ€™s energy budget from 1961 through 2008. (a) The top line indicates the integrated radiative forcing from wellâ€mixed greenhouse gases, ozone and solar changes. This radiative forcing is offset by the cooling due to stratospheric aerosols from volcanic eruptions (cyan), the warming of the upper (0 to 700 m) ocean (blue), the deep ocean (red) and energy absorbed in the melting of ice and warming the atmosphere and the land (green), and the change in outgoing radiation inferred from temperature change (brown). The integrated aerosol (and other) forcing is estimated as a residual (grey). (b) The inferred global averaged aerosol radiative cooling (red) is calculated for three different values of lambda (N = F âˆ’ Î»Î”T, where N is the net heat flux into the climate system, F is the radiative forcing, Î”T is the global averaged temperature change, and climate feedback parameter Î» is inversely related to climate sensitivity to a doubling of carbon dioxide). The equivalent results for the Gregory and Forster (2008) volcanic forcing and the present heat content estimates are shown by the blue line and shading. Results from previous aerosol forcing estimates of Gregory and Forster (2008) and Goddard Institute for Space Studies (GISS) forcing are shown in black solid and dashed respectively.
Factors reducing the uptake of solar energy by the Earth’s system
The changes in the Earth’s energy balance involves radiative forcing (F) from well-mixed greenhouse gases, ozone, fluctuations in solar radiation and volcanic aerosols.
- Well-mixed greenhouse gases, ozone and solar forcing would result in the storage of 1,735 Ã— 10**21 J of additional heat in the Earth’s system.
- However, only 250 Ã— 10**21 J of the forcing is stored in the Earth system. Over 90% of the additional heat is stored in the ocean with minor storage in the atmosphere and the solid earth and consumption in the melting of ice.
- Negative radiative forcing from volcanic aerosols offset about 320 Ã— 10**21 J of the greenhouse gas forcing.
- The remaining portion 1200 Ã— 10**21 J must be balanced by the sum of reflection of solar radiation by tropospheric aerosols and increased radiation from a warming Earth.
The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as âˆ’0.8 Â± 0.4 W/m**2 for the 1980s and early 1990s. The increases in the late 1990s is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity.
There are several potential contributions to the increased negative forcing (cooling).
- Decreases in stratospheric water vapour, which have not been taken into account, give a negative forcing and could explain 0.1â€“0.2 W/m**2 of this inferred change.
- Inadequate control of sulphur emissions since the early 2000s, particularly in China, would lead to a more negative aerosol forcing.
- Increases in sulphur emissions from developing nations, particularly in south and east Asia since 2000.
- A large pulse of organic carbon emissions in southâ€east Asia in 1997â€“1998 from forest fires.
- At the Mauna Loa Observatory, the increase in 15.8â€“33 km aerosol backscatter over 2000â€“ 2009 is an increased negative radiative forcing of about âˆ’0.15 W/m**2 over the decade.
- A series of moderate volcanic eruptions have increased stratospheric aerosol loading (not included in this study’s forcing time series) at heights greater than 20 km since 2002.
- An increased negative stratospheric forcing since 2006 of âˆ’0.1 W/m**2, resulting in a total value of âˆ’0.2 W/m**2 in 2009.