MASUDA Kooiti [Author].
We have an outlook that climate will significantly change during the 21st century, that the change includes rise of global mean surface air temperature, and that the main cause of the change is carbon dioxide (CO2) which comes from burning of fossil fuel by human beings. This outlook is called "anthropogenic global warming" (AGW), or casually just "global warming". The outlook of AGW is now shared by many scientists who study climate change. Let me call this position "the mainstream view" as a shorthand. This year (2007), the 4th Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) is being compiled [See Note]. The report of the Working Group 1 which deals with the physical scientific basis of climate change reveals that the mainstream view has become more certain than before. The consensus, however, does not mean agreements in details. Even the outlook of the rise of global mean surface air temperature has some range of uncertainty. Outlooks of changes of regional temperature and moisture have considerable amount of uncertainty.
In bookstores [in Japan], I find many books about global warming. Some do reflect the consensus (more-or-less) among experts. But, more eye-catching are statements like "Global warming will not happen" (let me call them "AGW-denialism") or "The scientific basis of the outlook of AGW is not reliable, and no one knows how the climate will be" ("AGW-skepticism") on one hand, and "Global warming will result in catastrophe of the ecosystem and of the human society" ("AGW-alarmism") on the other hand. Note, however, that the opinions are different, sometimes even contradictory, from an author to another within the same category in this tentative classification.
The motive why I write this essay is the hope for readers to understand the mainstream position of the science of climate change. Here, however, I would like to show not just the common view of the mainstream, but also my personal thoughts about the meaning of the AGW problem and about the role of science which study the issue.
It is obvious that the human society is constrained by the environment, though it is not completely determined by it. On the other hand, human activity affects the environment. Sometimes the environment as modified by human activity becomes more stringent for human lives. This is what the term "environmental issue" means, as I consider. We can find many examples of regional environmental issues. Since ancient times, destruction of forests, due to agriculture, pastoralism or urban fuel needs, often resulted in waste lands unsuitable for human settlements. In the modern times, the issue has extended to the whole earth, and we humans do not have a place to get away. Thus we have global environmental issues. (Here I include the issue of exhaustion of natural resources as well, since there is no clear boundary between it and the environmental issue, as we can understand from the example of deforestation.)
The cause-and-effect chain of AGW (Human beings burn fossil fuel, CO2 in the atmosphere increases, and the climate warms) is certainly a typical example problem among global environmental issues. But we shall think a while whether this is something worrisome. The level of global mean surface air temperature that is likely to occur a hundred year ahead is 2 to 5 degrees Celsius higher than the level before the industrial revolution. Though, according to current science, it exceeds the level that happened any time during the last 800,000 years, it is not an abnormal level when we look at the last 100,000,000 years. The rate of temperature rise such as 3 degrees Celsius per 100 years is likely to be abnormal, but such rate may have happened several times during the glacial period from 100,000 years ago to 10,000 years ago (though we cannot say it for sure because of uncertainty in estimates of global mean temperature of the geological past). For us humans, it would not be a crisis but just a call for migration if the total population were at most of the order of one million as it was in the glacial period. In the modern times, however, most of land of the world is divided into nations, most of land which can have some productive use is under property rights, but there is no authority which can plan and conduct relocation of population in the global scale. In this situation, change in the geographical distribution of food productivity is a worrisome issue, even if the total food productivity may not decrease.
It is remarked that AGW causes crises for the ecosystems and biodiversity of the earth. Though I am not an expert, I think it plausible that ecosystems of many places of the world such as coral reefs and alpine vegetation are in the state of crisis, and that human activity is an important factor of the crisis. I consider, however, that the primary factor among the human activity is land-use change including clearing of land for agriculture and urbanization, and that the factor of AGW is secondary though not negligible. If climate change happens alone, many (though not all) organisms would be able to adapt by migrating to the place where the climate becomes suitable for them. But, in reality, migration is hampered by cropland, pasture, and such constructions like automotive roads and dams. I think that AGW is a serious issue in this context because of the synergetic effect that the species whose habitat have been narrowed by land-use change are now exposed to climate change.
Another issue is that the human society in the modern times consumes a lot of non-renewable natural resources, in particular, energy resources. This situation is obviously not sustainable. It is not easy, however, to restructure the society into a sustainable one, since the modern economic principles have been developed since the latter half of the 18th century with the condition that fossil fuel is always available. Without the problem of AGW, we would certainly choose a policy to rely on coal for the coming century and then gradually plan for the next transition. But, the need to avoid AGW urges us to become independent of fossil fuel more quickly.
Thus, the issue of AGW is entangled with the issue of impact of land-use change on the ecosystems on one hand, and the issue of our dependence on fossil fuel on the other hand. In order to be sustainable, the human society must take a policy that does not cause crises in any aspect among these issues. Since the human impact on the environment already exceeds the range of resilience of the nature, our society must change so as to decrease the impact. Moreover, the places where the impact realizes are often in so-called developing countries, but the motive that causes the impact often comes from economic actors located in countries rich in monetary terms. I think that rich countries have responsibility to make their economic policy consistent with sustainability of the whole human society.
A helpful concept in this context is "ecological footprint" proposed by WACKERNAGEL and REES. These authors summed up the human impact on the environment in terms of the single physical dimension of area, and they demonstrated that the total impact already exceeded the total area of productive land of the world. This kind of quantitative indices have a merit that we can handle various impacts on the same table. On the other hand, they have limitation due to simplification. In particular, WACKERNAGEL and REES represent the impact of consumption of energy resources in terms of (estimated) area which is needed to absorb CO2. The actual impact is not only AGW.
When we think we should keep AGW small, the reason is not always the same, but probably part of it is a moral judgement that we should not cause trouble to the future generations. From the standpoint of this moral principle, we cannot endorse some of the proposed ways of mitigation of AGW. One of them is use of more nuclear power. We should evaluate the whole life cycle including production of fuel, construction of plants and handling of wastes, and it may turn out that the impact is smaller than the case of fossil fuel in terms of emission of CO2. But the way of disposal of the nuclear wastes is not established yet. Probably the only practical way is geological repositories, but they will be affected by local crustal movements whose prediction is very difficult. We should not seal them but we should keep active management for a long time. Thus, as we produce radioactive wastes, we force the future generations to bear the cost. Another one is sequestration of CO2. This is much safer material than radioactive wastes, but the amount is many orders-of-magnitude larger. CO2 must be kept sequestered for a time period of the order of 1000 years. I do not think that reliable sequestration is possible in such an active area of crustal movement as Japan. Even if it is possible, land use above the site of sequestration will be restricted for a long time.
Therefore, the energy resources on which the human society can depend in the future is limited to the renewable ones. Except for geothermal and tidal energy, they all are utilization of the flow of energy coming from the sun and going to the space: solar radiation, wind, hydropower, biomass fuel, etc. Even among them, we should avoid large dams and large-scale cultivation of fuel crops, because these have large impacts on ecosystems. Modern industrial societies have developed centralized systems which process large amount of energy at the central plants and then distribute it to many consumers. This mode is our adaptation to fossil fuel or nuclear power. In the future, our mode of production and consumption must adapt to such energy resources that have low spatial density and large temporal variability.
Maybe, opinions about AGW which are sold well tend to be extreme in either directions because they resonate in feeling of readers. As I suspect, many persons already sense that the mode of industry and living after the industrial revolution is not sustainable, and that AGW is one of the cues that urge us to get out of that mode. It may be psychologically reasonable then, that some people feel this inevitable transformation like "the end of the world" and become either pessimistic or indignant, and that some other people tend to subconsciously deny the existence of AGW.
Besides, we should not ignore those people who want to drive policies of governments to the direction which they desire. They use by choice such outlooks of climate change that are convenient to them. Policies about energy resources have many stakeholders all over the industries. In particular, in the United States of America, petroleum and coal companies sponsor many organizations which advertise opinions of various AGW-deniers and AGW-skeptics on pamphlets and web sites. I suspect that their purpose is to derail such policies that regulate CO2 emission. Most of the opinions which they promote are such ones that cannot stand against scientific criticism now, but it seems that their authors still believe that they tell the truth. Such organizations are formally charitable corporations ostensibly promoting public understanding of science, and therefore independent authors who want to take a neutral position also often trust and echo them. On the other side of the debate, people who want to promote use of nuclear power often emphasize AGW, and sometimes make use of AGW-alarmism.
Some of AGW-alarmists appeal about the graveness of human impacts on ecosystems based on good research of actual examples. They are precious warning. But when they attribute all the perils to AGW, their suggestions of countermeasures also concentrate on prevention of AGW, and they may tend to go towards such unsustainable directions as expansion of use of nuclear power. I hope that stories about the crisis of ecosystems should include how AGW works in synergy with other factors.
Scientific knowledge is hardly absolute truth. Correctness of scientific knowledge is always a tentative one. Science has, however, a built-in mechanism to improve its knowledge. The mechanism is based on "peer review" -- evaluation by peer scientists. It consists of, first, (usually anonymous) review of manuscripts before publication, and, second, open review of papers after publication (often in the form of another original paper). In order for peer review to work, scientists must learn and share the system of concepts of a discipline on one hand, and they must become able to criticize the works of their peers (possibly including their teachers) on the other hand. Science is based on tension between tradition and innovation.
Environmental sciences have larger uncertainty than those disciplines of science in which experiments with real physical bodies in controlled situation is possible. This difficulty has been overcome to a considerable degree, however, by way of numerical models, which are basically products of theories, as mentioned shortly.
Even then, the feasibility of prediction of a complex system, which contains unknown processes as well, is dubious. In the context of AGW in particular, we must predict the amount of CO2 emission in order to predict climate change. It is a difficult philosophical question whether we human beings really have free will. But anyway, it may be said that prediction of the future of a system which includes the society is a task which is out of capacity of science.
Nevertheless, despite science has uncertainty and despite it cannot make prediction (in the proper sense of the term), having scientific knowledge is not equal to knowing nothing at all. Science can often offer explanation of events that happened in the past. As for the future, it can tell what kind of events are likely to happen when several premises are postulated. ("Projection", to be explained shortly, is this type of activity.)
It may be reasonable to say that uncertain scientific knowledge is not strong as a basis of policies. But we cannot eradicate uncertainty. Therefore, insisting on this logic indefinitely may result in delaying policy-making indefinitely. The society needs to make decisions on policies from time to time taking account of both graveness of the problem and uncertainty of the understanding.
Some AGW-skeptics insist that the prevalent science of AGW is unreliable because it is distorted by politics.
It is true that IPCC is an intergovernmental organization. Also, governments of many countries including Japan promote national projects of climate change research whose immediate goal is showing up in the reports of IPCC. The situation is certainly different from the ideal type of academic science, that is, voluntary quest for truth. Also, some scholars of science studies suggest that science for policy is (or should be) "regulatory science" which has a different evaluative norm from that of academic science.
In reality, there are various practices distributed between the two ideal types of academic science and regulatory science. And the evaluative norm of IPCC is near that of academic science. It is partially because that IPCC was not given a authority to directly participate in politics but just to report comprehensive assessments of scientific knowledge. Moreover, IPCC adopts, as its mechanism of quality control, peer review as it is in academic science: first, what IPCC reviews are, in principle, peer-reviewed scientific papers; second, the drafts of its assessment reports are subject to reviews of other experts.
The situation of national priority projects, however, is not the same as academic science, in the sense that the basic theme of research is given in a top-down way, and that the deadline to answer is specified. It is reasonable to be anxious whether thoughts of researchers engaged in such projects may drift towards the desire of the sponsors. As far as I watch the contribution of Japan to the Working Group 1 of IPCC, however, no such biases seem apparent.
Academic science has such weakness that communication between different disciplines tends to be difficult though mutual criticism within each discipline may still work. The field of climate change research cannot be free from this kind of evil of specialization, but it is a field where its research activities tend to counteract the evil. Development of climate models requires collaboration between specialists in several different disciplines. Moreover, assessment of impacts of climate change requires mutual understanding between experts of far more disciplines.
The fact that CO2 in the atmosphere absorbs and emits infra-red radiation was demonstrated by laboratory experiments by TYNDALL in the mid-19th century. Quantitative evaluation of the processes based on the theoretical framework of quantum chemistry and precise laboratory experiments was made in the 1950s, and it is basically still valid now. Concerning how these processes affect the energy balance of the atmosphere and the earth's surface, general consensus among experts was obtained already in the 1970s.
An index often used is "the steady-state response to CO2 doubling", that is, the difference of temperature between two worlds, in one of which the CO2 concentration is fixed at some value (e.g. 300 ppm) for a very long time, and in the other it is fixed at the value twice as large as the former one. In a report of the U.S. National Academy of Sciences published in 1979, the value of the steady-state response to CO2 doubling was estimated to be between 1.5 and 4.5 degrees Celsius. This range of values happens to be considered valid now as well. The principal reason of this rather wide range is not the uncertainty in the effect of CO2 itself, but the uncertainty in how water vapor and clouds would behave in accordance to the change of temperature. Such uncertainty still remains, partly because it is still impossible to make global three-dimensional simulation of climate in which individual clouds are resolved (The performance of computers is still not sufficient). But I guess that further studies would not change the order-of-magnitude of the estimate of the steady state response.
The situation of the real world is different from the steady state response in that the amount of CO2 increases gradually. The atmosphere exchanges energy with the ocean. Since the ocean has orders-of-magnitude larger mass than the atmosphere, it takes much more time to warm the ocean than to warm the atmosphere. As studies done in 1980s revealed, the atmospheric temperature changes concurrently with the temperature of the upper part of the ocean where mixing by wind-driven circulation is dominant (up to several hundred meters deep), therefore its change lags the change of CO2 concentration in the atmosphere by several decades. On the other hand, the deep part of the ocean warms up more slowly with a time scale of one thousand years.
During the 1980s, the carbon cycle between the atmosphere, the ocean and the land was an enigma, but the amount of burning of fossil fuel and the amount of increase of CO2 in the atmosphere were well known. Therefore, with an assumption that the ratio between the two would remain the same, it was then already possible to calculate the likely course of future warming.
But CO2 is not a single factor that act to change the climate. The effect of aerosols (small solid or liquid particles suspended in the air) is another one. Among aerosols are sulfuric acid droplets made by oxidation of sulfur dioxide (SO2) which comes from burning of coal, petroleum etc. They act to lower the surface air temperature because they reflect sunlight effectively. In 1970s, some scientists thought that this effect would be stronger than the effect of CO2 so that the climate would cool. Later studies revealed that the aerosol effect is, though not negligible, in relative terms smaller than the effect of CO2.
Thus, in 1980s, more and more scientists became thinking that human activities are likely to warm the climate. Its impact on the ecosystem and agriculture was estimated, and it seemed that detrimental impact prevails if the amount of change exceeds some modest level. And it was considered that both efforts to adapt to climate change and efforts to prevent excessive warming would be necessary. Such arguments motivated the United Nations and in 1988 it decided to establish IPCC.
The arguments so far have been composed as theoretical outlooks about the future. But they are not powerful enough to influence politics. Fortunately or unfortunately, in early 1990s, more and more pieces of information suggest that temperature of the (then) recent decade is higher than before. Then the questions "Is it certain that the temperature has risen during the 20th century?" and "Can we attribute the warming to the increase of CO2?" were posed. Investigation of these questions involves not only theoretical researchers but also observation-based researchers. Also, these questions sound familiar to lay people. As the studies progress, confidence in the answers to these questions has increased. In the IPCC AR4 issued this year (2007), it is stated that "Warming of the climate system is unequivocal", and that "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations". This is now one of the arguments that support the outlook of global warming in the 21th century.
So far I have talked about "theoretical outlook", but actually it involves a kind of tools called "climate models". In a climate model, both theoretical and empirical knowledge are consolidated, but we can regard it essentially as a product of theoretical thoughts.
A climate model is a program which works on a computer. It computes step-by-step changes of motion as well as of temperature of the atmosphere and the ocean, both of which have 3 spatial dimensions. The skeleton of the model is a set of simultaneous equations that represent basic physical laws such as conservation of mass, conservation of energy and Newton's equation of motion. In developing a model, the order of magnitude of each term of the equations are evaluated. Such terms that have smaller order of magnitude may be omitted. But such terms that may have the same order of magnitude as the major ones are not omitted. Thus the resultant set of equations is too complex to be solved for a general solution. But still it is possible to obtain its numerical solution corresponding to specified initial condition and boundary condition. Also it often happens that it is practically impossible to know the numerical value of a certain term from physical laws only. In such cases, we use an empirical formula as a part of the model, while making sure that it does not violate physical laws. Such partial empirical modeling is called "parameterization". Such empirical formula usually contains numerical parameters. The values of the parameters are usually determined based on past experience, but they may also be tuned according to the results of simulations.
To evaluate performance of models, such experiments that reproduce the past history of climate are conducted. Though they do not comprise a proof that the models are correct, good achievements of the models have gradually risen experts' confidence on them. Some people criticize that parameter tuning of models makes their false apparent good performance, but that is not appropriate in this context. Unlike purely empirical models which are usual in social sciences, climate models are constrained by physical laws, and thus they cannot be fine-tuned to past experience.
Arguments of some AGW-skeptics are based on the idea that science cannot predict the future.
Actually, so-called "climate change prediction experiments" which are adopted by IPCC reports are not predictions in the proper sense of the word. IPCC AR4 calls them "projections" to make sure the distinction of the meaning.
A major set of projections that was conducted in expectation to be used by the IPCC AR4 was the 3rd phase of Coupled Model Intercomparison Project (CMIP3) which was a part of the World Climate Research Programme (WCRP). Prior to this, a working group of IPCC postulated several different scenarios of evolution of the human society, and estimated future time series of emission of CO2 etc. according to each of the scenarios. (This work was published in 2000. In it, such policy that deliberately control emission is not included.) Putting the emission scenarios to a numerical model of the carbon cycle between the atmosphere, the ocean and the land, the time series of CO2 concentration in the atmosphere according to each of the scenarios were obtained. In CMIP3, more than ten research groups put these concentration scenarios into their respective climate models and simulated climate change. The results differ from one scenario to another, and also from one model to another even with the same scenario.
The carbon cycle model which was used in an earlier step in the above procedure is by no means perfect. Also there must be some interaction between climate and carbon cycle in the real world. Accordingly, projections other than CMIP3 are being conducted, where the numerical models employed are so-called "earth system models" which incorporate changes in carbon cycle as well as climate.
On the other hand, experience of projections has revealed that, difference of climates between plausible scenarios is small, if we limit the time frame to 30 years or so from now. Thus it is considered that prediction of climate change within this time frame is feasible. Unlike weather prediction or seasonal climate prediction, climate change prediction will give seasonal statistical summaries representative of a time span of 10 years or so, rather than the weather of a specific day or the climate of a specific year.
Anyway, the society needs to make decision by using what science has found while being well aware of the limits of what science can do.
Extreme opinions about AGW by some scientists whose expertise is in different fields seem to me as based on decades old recognition of the state of the science concerning AGW. Some AGW-skeptics mention that scientists in 1970s were not able to make a meaningful prediction (or projection) of climate change. That is true, and also the core of the theory of AGW has not changed since then. But, as mentioned above, knowledge about the peripheral factors of the theory has advanced, and performance of numerical models has dramatically increased. Some AGW-deniers say that the "greenhouse effect" of CO2 is nearly saturated. That is the same as a sort of scientific view which was already rejected in 1970s, and we can learn how it was rejected from textbooks of the discipline [studies of radiative processes in atmospheric science].
On the other hand, some AGW-deniers and AGW-skeptics who have become interested in climate change in relatively recent times tend to discuss details in whether the alleged signals of warming found in observational data in the 20th century are significant, and how well numerical models reproduce those signals. These are discussions on smaller changes than those changes which are expected to happen in the 21st century (according to the "mainstream" outlook), so it is difficult to settle the debate whether they are signals or noise. As for an issue within science, it is a good thing to examine data in detail. But, what the society needs primarily for decision making is evaluation of theoretical outlooks for the 21st century. Discussion on detection and attribution of changes in the 20th century is useful just as one of the ways to evaluate the reliability of the theory-based framework.
The postulates of projections do not include exhaustively those factors of climate change which can happen in the future.
Results of projections show that climate will change relatively smoothly [if fluctuations are averaged out]. Some scientists, who either reconstruct (estimate) past climate or make theoretical investigations, suggest that climate may change more rapidly. Some of the suggestions (e.g. items 1, 2 and 3 of what follows) are possibilities of positive feedbacks (chains of causes and effects that act to amplify changes) which involve such processes that are not yet incorporated (or just being incorporated experimentally) in climate models which are employed in projections. Some others (e.g. items 4 and 5) are possibilities in which climate shifts to a much different state from the present state.
Note, however, that we cannot yet even reasonably guess the probability of any of these storylines coming true. Also, a person who points out a certain chain of causes and effects tend to emphasize his/her own storyline, but there may be other chains of causes and effects that have not come up to minds of anyone. Speaking in passing, we can regard such possibility that some negative feedbacks (chains of causes and effects that act to damp changes) alleviate AGW naturally as another "surprise" storyline.
Besides, there are possibilities of climate change by causes outside the system of climate. One example is that a massive explosion of a volcano injects SO2 in the stratosphere, and SO2 becomes sulfuric acid aerosols that reflect sunlight and thus cool the climate. Another is that the amount of energy which comes from the sun varies wildly.
It is prudent that we should prepare for the worst outcome. But we cannot take in account equally such storylines for which we cannot place even guesses of probability and such storylines that are based on model projections. Also, we can imagine many kinds of "worst" cases, and we do not know which one we should regard important. In this situation, I think that the human society should prepare for climate change in two tiers. The first is to prepare for probable changes, with reference to the "mainstream" projections together with their estimated range of uncertainties. Both the measures of adaptation to projected warming and the efforts to keep warming as small as possible should be included in this tier. The second is to further enlarge the adaptability of the society, taking various "surprise" climate changes (both by feedbacks and by external factors) into account as emergencies.
[Note]
IPCC is an organization established in 1988 under the United Nations
(in particular, the United Nations Environment Programme
and the World Meteorological Organization)
in order to synthesize scientific knowledge about climate change.
It has three major working groups based on the division of its tasks into
(1) how climate will change,
(2) how the changes will impact ecosystems and human society,
and (3) how we can mitigate detrimental impacts.
It issues assessment reports once in 5 or 6 years.
Its 4th Assessment Reports (AR4) are expected to be published within the year
2007,
and already their Summaries for Policymakers (SPMs)
as well as large part [all, by 2008] of their main texts
have been published on-line
and accessible by following links from the main WWW page of IPCC at
http://www.ipcc.ch/ .
Japanese translation of the SPMs are accessible from a WWW page
http://www.env.go.jp/earth/ipcc/4th_rep.html
maintained by the Ministry of Environment of Japan.
[Author] MASUDA Kooiti. Born in 1957. A scientist working on climate and water cycle. Doctor of Science in geophysics at the University of Tokyo. Presently a sub-group-leader of the Hydrological Cycle Research Program, Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology [Changed in April 2009 as follows: a senior scientist, Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology]. A co-author of "Kikô Hendô Ron" ("Climate Change", Iwanami Lectures in Earth and Planetary Science, Volume 11, 1996). A co-translator of the Japanese edition of Spencer WEART's "The Discovery of Global Warming".
The opinions stated in this text are personal opinions the author, and not necessarily representative of the organization he belongs to.