revisions. In a comprehensive approach, this might be offset against the effects of other greenhouse gases, thereby requiring weaker restrictions on their emissions. When ozone-depletion feedback is included, the strengthening of the protocol leads to a much smaller effect on 1990-2100 radiative forcing, a reduction of only ~0.1 W m2. This implies less latitude for easing restrictions on other greenhouse gas emissions if we are to achieve given goals of radiative forcing change.

8-10,29

Sulphate aerosols. The climatic importance of sulphate aerosols derived from man-made SO2 emissions has only recently been considered seriously There are three main effects1o, a reduction in incoming short-wave radiation under clear sky conditions 8.30 , a possible increase in cloud reflectivity due to sulphate aerosols acting as cloud condensation nuclei and a possible increase in cloud lifetimes". The first of these (the 'direct effect') has been best quantified by Charlson et al. who obtain a negative forcing of 1.07 W m2 in the Northern Hemisphere for global SO2 emissions of 71 Tg S yr ́1 (~64 Tg S yr ́1 in the Northern Hemisphere). The stated uncertainty is a factor of two. This negative forcing is of comparable magnitude to the greenhouse-gas-related positive forcing, ~2 Wm over the period of significant anthropogenic SO2 emissions, implying that aerosol changes may have noticeably offset the enhanced greenhouse effect in the Northern Hemisphere. The indirect effects through cloud albedo and lifetime changes are currently difficult to quantify reliably because of the uncertain link between changes in mass of sulphate aerosol and changes in number of cloud condensation nuclei", but they could add appreciably to the direct effect.

-2

Because of the wide uncertainty in the aerosol forcing, we use a range of possible values. As a best-guess value for the relationship between radiative forcing and emissions under the direct effect we use the results of ref. 8 and assume that a Northern Hemisphere emission level of 64 Tg S yr1 leads to a corresponding hemispheric radiative forcing of -1.07 W m2. We also assume that global emissions are always partitioned into 90% in the Northern and 10% in the Southern Hemisphere. As the estimate of the direct effect in ref. 8 assumes that emission sources are at present-day elevations, we need to account for changes in these elevations, the 'tall stack effect'. This change has increased the fraction of SO, that is oxidized to sulphate, currently 0.5 (ref. 8), whereas before 1950, it was probably - around one-third. We account for this effect by reducing the forcing by 30% before 1950, with the reduction factor decreasing to zero in 1970. For emissions after 1990, we assume forcing to be directly proportional to the anthropogenic component of SO emissions, with a 1990 global emissions level of 75 Tg S yr (refs 4, 20; see Table 1).

For the indirect effect, we assume that it is real but small, and of current (1990) magnitude equal to 20% of the direct effect. To quantify the link between SO2 emissions and indirect forcing more generally, we use a relationship suggested by Wigley' which leads to

AQ, In (1+E/E)

tially less than the direct forcing, the implied uncertainty in the former is much greater than in the latter. For 1990, we deduce a global-mean aerosol forcing range of -0.75±0.38 W m2. These values are much less than estimated in ref. 10, which gave best guesses of -1 W m2 for both direct and indirect effects separately, and a best-guess range of -1 Wm2 to -2 W m-2 for their sum. As shown in the next section, however, and as noted by Wigley29, aerosol forcing effects of this magnitude are almost certainly incompatible with observed hemispheric-mean temperature changes.

Total forcing projections. A summary of the radiative forcing projections over 1990-2100 is shown in Fig. 2. This gives two total forcing possibilities for each scenario: a base case which follows the methodology of IPCC90 and ignores CO2 fertilization feedback, ozone-depletion feedback and aerosol effects; and a 'best-guess' case, in which the two feedbacks are accounted for and in which the best-guess aerosol forcing is included. Numerical values for the year 2100 are summarized in Table

(5)

0.2

where E is the initial (natural) emissions level of sulphate precursors (taken as 42 Tg Syr', the midpoint of the range given by IPCC9220), and E is the anthropogenic emissions level. This formulation attempts to account for the nonlinear relationships between changes in initial emissions and final cloud condensation nuclei. For scenarios in which SO2 emissions grow (a, b, e and f), it leads to a much slower rate of increase after 1990 for the indirect effect than for the direct effect. Because the indirect effect is assumed to be a minor component in 1990, and because it becomes relatively less important subsequently, the results for total radiative forcing are insensitive to uncertainties in the AQ, formulation.

Finally, to account for uncertainties in the aerosol forcing we consider a range of possibilities, 50-150% of the combined aerosol forcing. As the indirect forcing is assumed to be substanNATURE VOL 357 28 MAY 1992

FIG. 3 Radiative forcing and temperature changes up to 1990. The panels show the effect of stratospheric-ozone-depletion feedback (compare curves I and II) and the additional negative forcing due to sulphate aerosols for low (L), middle (M) and high (H) estimates. Aerosol forcing was determined as a function of SO2 emissions as described in the text. The anthropogenic global emissions history assumed was based on ref. 46 to 1975 and tied to the IPCC92 best-guess estimate of 75 Tg Syr1 in 1990. (We used a piecewise linear fit to the data through zero in 1861, 35 Tg Syr1 in 1953 and 68 Tg Syr1 in 1973. The implied lower trend over 1973-90 is uncertain, but in accord with data for Europe and with the recent reduction in emissions growth in the USA.) The lower panels show both modelled and observed temperatures, the latter from IPCC92" smoothed with a 10-year gaussian filter. Model projections use best-guess parameters.

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For comparison, the 2100 values for the IPCC90 scenarios are also shown in Fig. 2.

The most important points to, note are the substantially reduced forcing in cach best-guess case relative to the corresponding base case; the wide range of forcing possibilities, all corresponding to the same basic ‘existing policies' assumption; and the reduced range of uncertainty in the best guess compared with the base case, largely because the high forcing cases have their larger CO2 effects offset more by aerosol forcing. The reduced forcing compared with what would have been obtained using the IPCC90 methodology arises primarily from our inclusion of CO2 fertilization feedback and sulphate aerosol forcing. As can be seen from Table 1, the aerosol effect depends on the scenario: IS92b is similar to IS92a, scenarios e and f have larger negative aerosol forcing contributions, whereas c and d have small positive contributions. For the best-guess case, total forcing under the c and d scenarios is below the (policydriven) IPCC90 B scenario. None of the IS92 scenarios, however, approaches the very low forcing projections of IPCC90 scenarios C and D.

FIG. 4 Radiative forcing, global-mean temperature and global-mean sea level projections for emissions scenario IS92a. For forcing, the base case follows IPCC90 methodology (curve 1). Curve II includes CO2 fertilization feedback in calculating concentration trends. Curve III, slightly below II, additionally accounts for ozone-depletion feedback. L, M and H refer to low, middle and high aerosol forcing possibilities. Temperature and sea-level results are shown only for the base (dashed curves) and best-guess case forcings (full curves). Low, middle and high estimates are given (denoted L, M, H). For temperature, these are obtained using AT2, 1.5, 2.5 and 4.5 °C with other model parameters kept at their best-guess values. For sea level, corresponding thermal expansion contributions are combined with ice-melt values based on low, middle and high model parameter values. For comparison, the equivalent 2100 low, middle and high estimates of temperature and sea level change for the IPCC90 Business as Usual scenario (SA90) are shown in the right (denoted A(L), A(M) and A(H)), with SA90 forcing shown by A in the top panel.

298

Temperature changes to 1990

Temperature projections for the six IS92 scenarios were made using our upwelling-diffusion energy-balance climate model3, as used in IPCC90. Before describing the projections beyond 1990, however, we consider changes to 1990, as these affect the range of possible sulphate aerosol forcings and have implications for the climate sensitivity.

Climate models of the type used here incorporate very simplified representations of the physical processes involved. Nevertheless, they seem to simulate well the responses to external forcing produced by more complex models. They account for the lag effect of oceanic thermal inertia, and they allow one to assess the sensitivity of the results to model uncertainties, in particular to uncertainties in the climate sensitivity. The present model additionally differentiates between the hemispheres and so permits the consideration of forcing changes specific to one hemisphere, as is necessary if we are to account for the effects of sulphate aerosol forcing.

The most important model parameter is the climate sensitivity which is specified by the equilibrium global-mean temperature change for a CO2 doubling. Largely on the basis of general circulation model experiments, this temperature change, AT2x. is thought to lie in the range 1.5-4.5 °C, with a 'best-guess' value of 2.5 °C (ref. 1). The other parameters affecting the model's response are the mixed-layer depth (h), the oceanic vertical diffusivity (K), the upwelling rate (w) and the temperature change of high-latitude sinking water relative to the global-mean change (#). Sensitivity to these parameters is less than to AT2x (ref. 35). We assume h=90 m, K = 1 cm2s', w = 4 m yr1 and

0.2. Slightly different values were used in IPCC90. The set of values used here is justified more fully in ref. 36. The greatest difference from IPCC is in our choice of # (IPCC90* used π= 1). Our choice of a lower value is supported by Schlesinger and Jiang. Smaller produces larger surface warming and smaller oceanic thermal expansion. The net effect on sea level change is a small reduction.

Figure 3 shows the radiative forcing to 1990 and the corresponding modelled temperature changes, both for the global mean and for the Northern Hemisphere. Modelled temperature changes are shown only for AT2 = 2.5 °C. For comparison, observed temperature changes, as recently updated by IPCC", are also shown.

If ozone-depletion seedback and best-guess sulphate aerosol effects are included, then the total forcing change to 1990 is reduced markedly (by ~1 Wm2 or 40%) from the IPCC90 estimate". This brings the predictions of temperature change using AT2x=2.5 °C into closer agreement with observations. In the absence of these two factors, the modelled changes with AT2 = 2.5 °C are appreciably larger than the observed changes; the value of AT2x giving the best fit to the 1880-1990 warming is only 1.5 °C (compared with 1.4°C in ref. 36, which used slightly different temperature data). For the case with ozonedepletion feedback and best-guess aerosol forcing, the modelled changes using AT2x=2.5 °C are noticeably less than the observed changes. The required sensitivity to obtain the best fit to the observations is AT2, -3.4 °C. If only best-guess aerosol forcing is included (without ozone-depletion feedback), our best-guess empirical value for the sensitivity is 3.0 °C (compare with ref. 40 for similar assumed forcing). The inclusion of the aerosol forcing flattens the model-projected warming trend over 1950-70, especially in the Northern Hemisphere, in qualitative agreement with observations.

Figure 3 also indicates the possible range of values for the negative aerosol forcing. Our largest negative values, which are at the smaller end of the range given by Charlson et al.1⁄4o, lead to an overall negative forcing trend over 1860-1980 in the Northern Hemisphere. As a consequence, the modelled temperature change for the Northern Hemisphere over this period is much less than observed, and, more important, there is a marked difference in warming between the Northern and

NATURE VOL 357 28 MAY 1992

Southern hemispheres which is not evident in the observations. It is possible that differential warming has been masked by natural variability, but this seems unlikely. On empirical grounds, therefore, and in accord with refs 29 and 40 and three-dimensional modelling results", our choice for the range of likely aerosol forcing is more defensible than the values suggested in ref. 10 which were based on a simpler onedimensional analysis.

Future temperature and sea level

Projections of temperature change are obtained by running the climate model with the various past forcing possibilities combined with consistent projected forcings to 2100. The number of model runs to be made is determined by the product of the number of future emissions scenarios (6), the number of forcing projections for each scenario (12, see below) and the number of sets of model parameters (3, see below). For each scenario, we have considered forcing projections with and without CO, fertilization feedback, with and without ozone-depletion feedback, and for low, middle and high aerosol forcing. This set of 12 possibilities gives an idea of the uncertainties in future forcing under any particular emissions scenario, although it is not a comprehensive analysis of uncertainty. For the climate model, we have carried out simulations for different climate sensitivity values, AT2 = 1.5, 2.5 and 4.5 °C. As noted above, the sensitivity of the results to other model parameters is less than to AT2, (see ref. 35) so these have been kept at their best-guess values. Only a selection of results is presented here.

The climate model also produces results for global-mean thermal expansion of the oceans, a principal component of future sea-level rise. To obtain total sea-level rise, meltwater contributions are added from separate, highly aggregated models representing small glaciers, the Greenland ice sheet and Antarctica, each driven by the global-mean temperature projection. The ice-melt models are similar to those used by IPCC90 (which we developed in conjunction with Warrick and Oerlemans), with improvements as described in refs 35 and 42. Ice-melt uncertainties are accounted for by using a range of values for ice-melt model parameters 35.42.

Detailed results for IS92a are given in Fig. 4. This is the scenario most nearly equivalent to SA90. The upper panel shows the sequential reductions in forcing that arise from including a CO2 fertilization effect (I to II), accounting for ozonedepletion feedback (II to III) and accounting for the negative forcing due to sulphate aerosols (III to L, M or H). Including CO, fertilization to balance the contemporary carbon budget reduces the 1990-2100 forcing change by ~11% (compared with

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a maximum of 22% for IS92c and a minimum of 7% for IS92e). The effect of including stratospheric-ozone-depletion feedback on post-1990 forcing is negligible (in all scenarios), whereas aerosols reduce the IS92a forcing by a further 6-19% relative to the base case. Overall, the best-guess forcing for IS92a is -19% below the base case in the year 2100 (see Table 1 for results for other scenarios).

Low, middle and high temperature and sea-level projections for IS92a are shown in the lower panels of Fig. 4. For temperature, the warming over 1990-2100 is between 1.7 °C and 3.8 °C for the best-guess forcing case (compared with a warming range of 2.1 °C to 5.0 °C for base-case forcing). The corresponding sea-level rise lies between 15 and 90 cm for best-guess forcing and between 22 and 115 cm for base-case forcing. Temperature and sea-level change projections based on best-guess forcing are 20-24% and 21-30%, respectively, below those for base-case forcing.

The most direct comparison that can be made with IPCC90 is between IS92a and the SA90 results. The latter results are shown (for the year 2100 only) on the right-hand side of Fig. 4. IS92a and SA90 are comparable only in a limited sense, and differences arise because of differing emissions projections, revised methods for calculating the implied concentration and radiative forcing changes, changes in the assumed best-guess climate model parameters, and improvements in the ice-melt models. For temperature changes over 1990-2100, the IPCC90 best guess for SA90 (based on AT2, = 2.5 °C) was 3.3 °C. Our corresponding best guess for IS92a is 2.5 °C. For sea level, the equivalent projected changes are: SA90, 66 cm; IS92a, 48 cm.. These are substantial reductions, but the projected changes in global-mean temperature and sea level are still very large. For comparison, the warming corresponds to a rate roughly five times that observed over the past century, and the sea level rise is at a rate roughly four times that estimated for the past century. The uncertainty ranges for these projections are large, and they are higher in the present results than in IPCC90.

Figure 5 summarizes temperature and sea-level projections for all scenarios for the overall best-guess cases (AT2 = 2.5 °C, best-guess ice-melt model parameters, and best-guess forcing). IPCC90 best-guess cases are also shown (2100 values). For the IS92 scenarios, the differences in temperature and sea level projections up to 2050 are fairly small, less than the forcing differences between scenarios (see Fig. 2). For the next 60 years or so, future temperature and sea-level changes are, therefore, relatively insensitive to the uncertainties in future emissions. This means that the 1592a results given in Fig. 4 can be taken as representative of all the emissions scenarios until about 2050.

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These projections are subject to other large uncertainties, particularly uncertainties in the climate sensitivity (Fig. 4). Reducing the error bounds on the climate sensitivity should therefore have high priority in greenhouse research..

Conclusions

These concentration, radiative forcing, temperature and sealevel projections are meant to complement the scientific update? of the 1990 IPCC report'. We have addressed the new scientific issues raised in this update, the effects of CO2 fertilization on CO2 concentration projections, the possible negative feedback on halocarbon radiative forcing due to stratospheric ozone depletion, and the negative forcing due to sulphate aerosols. Largely because of these factors (specifically, the first and third), future projections of global-mean temperature and sea level change are reduced markedly (by 20-30%) below the equivalent projections made using the IPCC90 methodology (compare, for example, base-case and best-guess estimates in Fig. 4). But our projections, by including the effects of sulphate aerosols, introduce an additional complication: a reversed and enhanced differential in the rates of warming between the hemispheres. This could have an important influence on the climate system beyond the direct greenhouse effects currently considered by general circulation climate models.

The following important results have emerged. First, the effects so far of ozone-depletion feedback and sulphate aerosol forcing bring the best-guess empirical estimate of the climate sensitivity (AT2x = 3.4 °C) into closer agreement with the range of estimates based on general circulation climate model results. If both of these factors are ignored, the best-guess empirical sensitivity is only 1.5 °C. The improved agreement here is encouraging, but it should not be over-interpreted. If other uncertainties are taken into account, then one finds a very wide range of AT2x values to be compatible with observed temperature changes.

This point reinforces our second result, namely that uncer tainty in future greenhouse-gas-related temperature and sealevel changes is strongly affected by the climate sensitivity. Reducing uncertainties in this parameter should have high

priority, although the best strategy for this is far from clear. The main reason for our conclusion here is that projections of temperature and sea-level changes out to 2050 have only a small range for any given AT2, value, but differ widely according to the chosen value of AT2. The similarity between scenarios should not be interpreted as implying a general insensitivity to future emissions trajectories. Rather, it emphasizes the very long-term nature of the greenhouse problem. A more salutory result is the fact that, beyond the middle of next century, projections for the various scenarios diverge rapidly. Inertia in population growth and socio-economic systems would make it difficult to shift from one scenario to another.

Climate sensitivity is not the only uncertainty that needs to be reduced. In the scenarios in which SO2 emissions increase markedly (a, b, e and f), an important additional uncertainty arises through not being able to quantify the effect of aerosols to better than around ±50%. In scenario IS92a, for example, this translates to an uncertainty of ±6% in total forcing, ±8% in temperature change and ±9% in sea-level change (over 902100). Uncertainties arising from our inadequate understanding of the global carbon cycle, judged from a comparison of results with and without CO2 fertilization feedback, are of comparable magnitude.

The projections presented here show only the anthropogenic component of future change, and natural variability will be superimposed on this. Natural variability includes both internally generated variability“ and the effects of external forcing agents, the latter including short-term factors like the recent volcanic eruption of Mount Pinatubo. The expected anthropogenic rates of change, however, are much greater than the corresponding natural rates of change over intervals of 30 years or longer, becoming increasingly so as the interval increases. The reduced rates of warming and sea-level rise that we have obtained compared with IPCC90 are still four to five times those that have occurred over the past century. Such changes are certain to present a considerable challenge for humanity.

ט

T. M. L. Wigley and S. C. B. Raper are at the Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK.

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ACKNOWLEDGEMENTS. This work was supported by grants from the US Department of Energy. Atmospheric and Climate Research Divison, the Commission of the European Communities (DGXII), and the UK Department of the Environment Comments by K. P. Shine and P. M. Kelly helped to improve the manuscript. Computer-generated diagrams were produced by M. Salmon.

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TAKE A GOOD LOOK AT THE GRAPH ON THIS page, reproduced from a report that appears on page 698. It's giving climatologists goose bumps. The curves show that when the interval between peaks in sunspot abundance began shortening at the end of the past century, the Northern Hemisphere began to warm. When the sunspot cycle stopped shortening and began lengthening around 1940, the temperature peaked and began falling. And when the solar cycle length started shortening again in the 1960s, temperature turned around too. The close association of these two curves is the most striking correlation ever found between climate and small variations in solar activity-and the strongest sug gestion ever of a causal link.

"If it's correct," says atmospheric scientist Keith Shine of the University of Reading, "we have to change our view of climate fundamentally. It's an incredible correlation; it would imply that almost nothing else [beside solar variation] is important in the climate system." Even greenhouse warming would have played little role in the 0.5°C warming of the last century-which is not to say that it couldn't be important soon. Temperature

researchers will assume just that, given the long, checkered history of the search for sun-climate relations.

"We have seen a number of these that haven't worked out," says John Eddy of the University Corporation for Atmospheric Research in Boulder, Colorado, who has studied possible sun-climate connections on time scales of centuries. "We've been fooled so many times that we should be careful." One memorable example he cites is a stack of 680-million-year-old sediments that to believers and even some skeptics seemed to record climate variations on a timetable uncannily similar to various sunspot cycles (Science, 3 September 1982, p. 917). But then the bubble burst: It turned out that the layers had recorded nothing more than the cycles of the tides (Science, 18 November 1988, p. 1012).

In searching for their new sun-weather link, Eigil Friis-Christensen and Knud Lassen of the Danish Meteorological Institute faced the same handicaps as their predecessors: lack of good records of either climate or the activity of the sun. The researchers used the best available record of global temperature, but it covers only the land surface of the North

Year

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ern Hemisphere and goes back only

to about 1870. The risk in using such a short record is the greater likelihood that, just by chance, it will seem to vary in concert with some change on the sun.

The two researchers also lacked any direct indicator of the sun's total energy output, the most likely driving force of any sun-rc

lated climate change. Changes in solar irradiance, after all, have been accurately monitored for only the last 10 years. So, like others in search of a sun-climate link, FriisChristensen and Lassen had to use a stand

in. Previous workers have tried to gauge the sun's activity by such measures as the height of the peaks in the sunspot cycle, the frequency of auroras, and the variation in the amount of carbon-14 produced in the upper atmosphere and trapped in tree rings. Friis

Christensen and Lassen opted for the varying length of the sunspot cycle simply because it worked out and because it seems to track long-term variations in the solar wind-a direct, though feeble, part of the sun's energy output.

But it's far from clear that sunspot-cycle length actually changes in concert with any solar variation strong enough to drive climate change. And the fact that it is only the latest in a long line of solar-output proxies that have been tried doesn't inspire contidence. After so many rolls of the dice, solge climatologists complain, a lucky nuntber might finally have come up. "The more parameters you look at," notes climatologist David Parker of the Hadley Center for Climate Prediction and Research in Bracknell, England, "the more likely you are to find one that fits the global temperature curve [by chance."

If such skepticism weren't enough, FriisChristensen and Lassen have another problem to overcome in gaining acceptance for their correlation: Climate researchers can't explain how known solar variability might affect climate in the first place. The variations in solar output traced in recent years are too small to account for the temperature changes of the past century, and so far solar physicists haven't come up with any mechanism for larger swings in output. Some researchers have talked about the possibility that the upper atmosphere somehow amplifics the effects of the known changes in irradiance or other solar variations, but the search for such an atmospheric amplifier has been going on for decades without success.

After voicing all their doubts about the report, though, some researchers admit that they are mightily impressed by the close intertwining of the two curves. They have a correlation coefficient of 0.95, probably the highest ever found in this sort of work. Even Parker says he is impressed, though cautious. And Eddy goes further: "The fit is so good," he says, that "the burden of proof that something's wrong almost rests with any detractors." The usual qualifications must still apply, he says, but, "I'll still make a bet they're on to something."

Even so, it could be a long time before anyone proves it. Decades more of irradiance measurements would be required to show that the sun's output can vary enough on long time scales to have caused the halfdegree warming of the past century. Until then, researchers are stuck in limbo. Metco

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