Geoscience Reference
In-Depth Information
In search of the signal of climate change, a recent NASA study has attempted to do
platforms, radiative modelling, and a whole raft of assumptions and uncertainties. That
said, the results are viewed as the most meaningful way to estimate the role and context of
CO
2
in the atmosphere. Figure 2 shows the estimated global fluxes of the surface energy
balance.
The 24-year record of longwave radiation is shown in the middle graph—what is
immediately apparent is that there are large year to year variations. Major positive
variations are linked to particularly strong El Niño events. Underlying this variability, one
can perceive a relatively minor general increase in longwave flux, however it is small
relative to the naturally occurring variability within the series. It is worthwhile to note at
this point that atmospheric carbon dioxide rose by approximately 25 per cent over this
period—the longwave response to this (the actual mechanism of climate change) appears
rather muted if not entirely underwhelming.
Of even greater significance is the shortwave radiation (shown in the top graph). This
is the amount of solar radiation (sunshine) received at the surface and so is primarily
influenced by global cloudiness but also dust and aerosols (note the two to three-year
decline in shortwave following the 1991 eruption of Mt. Pinatubo). There are a number of
aspects of Figure 2 that are of importance to note.
First of all, the variability in the incoming shortwave is much greater than the
variability in the longwave. In fact, it is approximately twice as large. Consequently,
the shortwave variability is clearly more important to the overall energy balance. Also
important to note is the decadal timescales of variability—in particular, there appears to
have been a substantial declining trend since around 2000.
Finally, the shortwave variability shows no apparent correlation with the longwave
variability. Consequently, changes in longwave radiation do not appear to have any
influence on global cloudiness which has a larger influence on the overall energy balance
on decadal timescales. In Figure 2, the global net radiation anomalies are shown in the
bottom graph in comparison to the global temperature record shown in the top graph.
The net imbalance appears to have been negative since 2000, during which time global
temperatures have not risen.
