Hansen et al: Global climate changes as forecast by GISS 3D model

Hansen's 1988 paper that Pat Michaels misrepresented in testimony is not available online. I've put some extracts here.

Hansen, J., I. Fung, A. Lacis, D. Rind, Lebedeff, R. Ruedy, G. Russell, and P. Stone 1988. Global climate changes as forecast by Goddard Institute for Space Studies three-dimensional model. J. Geophys. Res. 93, 9341-9364.

Abstract

We use a three-dimensional climate model, the Goddard Institute for Space Studies (GISS) model II with 8° by 10° horizontal resolution, to simulate the global climate effects of time-dependent variations of atmospheric trace gases and aerosols. Horizontal heat transport by the ocean is fixed at values estimated for today's climate, and the uptake of heat perturbations by the ocean beneath the mixed layer is approximated by vertical diffusion. We make a 100-year control run and perform experiments for three scenarios of atmospheric composition. These experiments begin in 1958 and include measured or estimated changes in atmospheric CO2, CH4, H2O, chlorofluorocarbons (CFCs) and stratospheric aerosols for the period from 1958 to the present. Scenario A assumes continued exponential trace gas growth, scenario B assumes a reduced linear linear growth of trace gases, and scenario C assumes a rapid curtailment of trace gas emissions such that the net climate forcing ceases to increase after the year 2000. Pricipal results from the experiments are as follows: (1) Global warming to the level attained at the peak of the current interglacial and the previous interglacial occurs in all three scenarios; however, there are dramatic differences in the levels of future warming, depending on trace gas growth. (2) The greenhouse warming should be clearly identifiable in the 1990s; the global warming within the next several years is predicted to reach and maintain a level at least three standard deviations above the climatology of the 1950s. (3) Regions where an unambiguous warming appears earliest are low-latitude oceans, China and interior areas in Asia, and ocean areas near Antarctica and the north pole; aspects of the spatial and temporal distribution of predicted warming are clearly model-dependent, implying the possibility of model discrimination by the 1990s and thus improved predictions, if appropriate observations are acquired. (4) The temperature changes are sufficiently large to have major impacts on people and other parts of the biosphere, as shown by computed changes in the frequency of extreme events and comparison with previous climate trends. (5) The model results suggest that some near-term regional climate variations, despite the fixed ocean heat transport which suppresses many possible regional climate fluctuation; for example, during the late 1980s and the 1990s there is a tendency for greater than average warming in the southeastern United States and much of Europe. Principal uncertainties in the predictions involve the equilibrium sensitivity of the model to climate forcing, the assumptions regarding heat uptake and transport by the ocean, and the omission of other less-certain climate forcings.

4. RADIATIVE FORCING IN SCENARIOS A, B AND C

4.1. Trace Gases

We define three trace gas scenarios to provide an
indication of how the predicted climate trend depends upon
trace gas growth rates. Scenario A assumes that growth
rates of trace gas emissions typical of the 1970s and 1980s
will continue indefinitely; the assumed annual growth
averages about 1.5% of current emissions, so the net
greenhouse forcing increases exponentially. Scenario B has
decreasing trace gas growth rates, such that the annual
increase of the greenhouse climate forcing remains approxi-
mately constant at the present level. Scenario C drastically
reduces trace gas growth between 1990 and 2000 such that
the greenhouse climate forcing ceases to increase after 2000.
The range of climate forcings covered by the three
scenarios is further increased by the fact that scenario A
includes the effect of several hypothetical or crudely
estimated trace gas trends (ozone, stratospheric water vapor,
and minor chlorine and fluorine compounds) which are not
included in scenarios B and C.

These scenarios are designed to yield sensitivity experiments for a broad range of future greenhouse forcings.
Scenario A, since it is exponential, must eventually be on
the high side of reality in view of finite resource constraints and environmental concerns, even though the growth of emissions in scenario A (≈1.5% yr^-1) is less than
the rate typical of the past century (≈4% yr^-1). Scenario C
is a more drastic curtailment of emissions than has generally
been imagined; it represents elimination of chlorofluorocarbon (CFC) emissions by 2000 and reduction of CO, and
other trace gas emissions to a level such that the annual
growth rates are zero (i.e., the sources just balance the
sinks) by the year 2000. Scenario B is perhaps the most
plausible of the three cases.

The abundances of the trace gases in these three
scenarios are specified in detail in Appendix B. The net
greenhouse forcing, ΔT₀, for these scenarios is illustrated in
Figure 2; ΔT₀ is the computed temperature change at equilibrium (t → ∞) for the given change in trace gas abundances,
with no climate feedbacks included [paper 2]. Scenario A
reaches a climate forcing equivalent to doubled CO, in
about 2030, scenario B reaches that level in about 2060, and
scenario C never approaches that level. Note that our
scenario A goes approximately through the middle of the
range of likely climate forcing estimated for the year 2030
by Ramanathan et at. [1985], and scenario B is near the
lower limit of their estimated range. Note also that the
forcing in scenario A exceeds that for scenarios B and C
for the period from 1958 to the present, even though the
forcing in that period is nominally based on observations;
this is because scenario A includes a forcing for some
speculative trace gas changes in addition to the measured
ones (see Appendix B).

Our climate model computes explicitly the radiative
forcing due to each of the above trace gases, using the
correlated k-distribution method [paper 1]. However, we
anticipate that the climate response to a given global
radiative forcing ΔT₀ is similar to first order for different
gases, as supported by calculations for different climate
forcings in paper 2. Therefore results obtained for our
three scenarios provide an indication of the expected
climate response for a very broad range of assumptions
about trace gas trends. The forcing for any other scenario
of atmospheric trace gases can be compared to these three
cases by computing ΔT₀(t) with formulas provided in
Appendix B.

4.2. Stratospheric Aerosols

Stratospheric aerosols provide a second variable climate
forcing in our experiments. This forcing is identical in all
three experiments for the period 1958-1985, during which
time there were two substantial volcanic eruptions, Agung in
1963 and El Chichon in 1982. In scenarios B and C,
additional large volcanoes are inserted in the year 1995
(identical in properties to El Chichon), in the year 2015
(identical to Agung), and in the year 2025 (identical to El
Chichon), while in scenario A no additional volcanic aerosols
are included after those from El Chichon have decayed to
the background stratospheric aerosol level. The stratospheric aerosols in scenario A are thus an extreme case,
amounting to an assumption that the next few decades will
be similar to the few decades before 1963, which were free
of any volcanic eruptions creating large stratospheric optical
depths. Scenarios B and C in effect use the assumption
that the mean stratospheric aerosol optical depth during the
next few decades will be comparable to that in the volcanically active period 1958-1985.

The radiative forcing due to stratospheric aerosols
depends upon their physical properties and global distribution. Sufficient observational data on stratospheric
opacities and aerosol properties are available to define the
stratospheric aerosol forcing reasonably well during the past
few decades, as described in Appendix B. We subjectively
estimate the uncertainty in the global mean forcing due to
stratospheric aerosols as about 25% for the period from 1958
to the present. It should be possible eventually to improve
the estimated aerosol forcing for the 1980s, as discussed in
Appendix B.

The global radiative forcing due to aerosols and greenhouse gases is shown in the lower panel of Figure 2.
Stratospheric aerosols have a substantial effect on the net
forcing for a few years after major eruptions, but within a
few decades the cumulative CO₂/trace gas warming in
scenarios A and B is much greater than the aerosol cooling.

5. TRANSIENT SIMULATIONS

5.1. Global Mean Surface Air Temperature

The global mean surface air temperature computed for
scenarios A, B, and C is shown in Figure 3 and compared
with observations, the latter based on analyses of Hansen
and Lebedeff [1987] updated to include 1986 and 1987 data.
Figure 3a is the annual mean result and Figure 3b is the 5-year running mean. In Figure 3a the temperature range
0.5°-1.0°C above 1951-1980 climatology is noted as an
estimate of peak global temperatures in the current and
previous interglacial periods, based on several climate
indicators [National Academy of Sciences (NAS), 1975];
despite uncertainties in reconstructing global temperatures
at those times, it is significant that recent interglacial
periods were not much warmer than today.

Interpretation of Figure 3 requires quantification of the
magnitude of natural variability in both the model and
observations and the uncertainty in the measurements. As
mentioned in the description of Figure 1, the standard
deviation of the model's global mean temperature is 0.11°C
for the 100-year control run, which does not include the
thermocline. The model simulations for scenarios A, B, and
C include the thermocline heat capacity, which slightly
reduces the model's short-term variability; however, judging
from the results for scenario A, which has a smooth
variation of climate forcing, the model's standard deviation
remains about 0.1°C. The standard deviation about the 100-year mean for the observed surface air temperature change
of the past century (which has a strong trend) is 0.20°C;
it is 0.12°C after detrending [Hansen et al., 1981]. The
0.12°C detrended variability of observed temperatures was
obtained as the average standard deviation about the ten
10-year means in the past century; if, instead, we compute
the average standard deviation about the four 25-year
means, this detrended variability is 0.13°C. For the period
1951-1980, which is commonly used as a reference period,
the standard deviation of annual temperature about the 30-year mean is 0.13°C. It is not surprising that the variability of the observed global temperature exceeds the variability in the GCM control run, since the latter contains
no variable climate forcings such as changes of atmospheric
composition or solar irradiance; also specification of ocean
heat transport reduces interannual variability due to such
phenomena as El Nino/Southern Oscillation events. Finally,
we note that the lσ error in the observations due to
incomplete coverage of stations is about 0.05°C for the
period from 1958 to the present [Hansen and Lebedeff,
1987], which does not contribute appreciably to the variability (standard deviation) of the observed global temperature. We conclude that, on a time scale of a few decades
or less, a warming of about 0.4°C is required to be significant at the 3σ level (99% confidence level).

There is no obviously significant warming trend in either
the model or observations for the period 1958-1985. During
the single year 1981, the observed temperature nearly
reached the 0.4°C level of warming, but in 1984 and 1985
the observed temperature was no greater than in 1958.
Early reports show that the observed temperature in 1987
again approached the 0.4°C level [Hansen and Lebedeff,
1988], principally as a result of high tropical temperatures
associated with an El Nino event which was present for the
full year. Analyses of the influence of previous El Ninos on
northern hemisphere upper air temperatures [Peixoto and
Oort, 1984] suggest that global temperature may decrease in
the next year or two.

The model predicts, however, that within the next several
years the global temperature will reach and maintain a 3σ
level of global warming, which is obviously significant.
Although this conclusion depends upon certain assumptions,
such as the climate sensitivity of the model and the absence
of large volcanic eruptions in the next few years, as
discussed in Section 6, it is robust for a very broad range
of assumptions about CO₂ and trace gas trends, as illustrated in Figure 3.

Another conclusion is that global warming to the level
attained at the peak of the current interglacial and the
previous interglacial appears to be inevitable; even with the
drastic, and probably unrealistic, reductions of greenhouse
forcings in scenario C, a warming of 0.5°C is attained
within the next 15 years. The eventual warming in this
scenario would exceed 1°C, based on the forcing illustrated
in Figure 2 and the feedback factor f ≈ 3.4 for our GCM
[paper 2]. The 1°C level of warming is exceeded during the
next few decades in both scenarios A and B; in scenario A
that level of warming is reached in less than 20 years and
in scenario B it is reached within the next 25 years.