Part I: This is the first of a three-part series on global climate science and its relationship to global warming. Part II will focus on sea level rise and the Antarctic contribution. Part III will deal with atmospheric and oceanic circulation.
Global warming, a possible environmental problem, is a most unique issue. First, its principle actor, carbon dioxide (CO2), is at today's concentration, more of a resource than a problem. For example, there are many references on increased grain yields and enhanced forest growth under elevated CO2 atmospheres. Next, is the problem that both climate (in general) and temperature (in particular), have been changing slowly for ever.
This begs the question. If we are seeing any warming today - and that is uncertain - is it due to Mother Nature or to society? Finally, the principle impact of a modestly higher temperature may not be bad. There are many references illustrating that societies do better during warming periods versus colder eras.
An understanding of natural climate variation, whether random or cyclical, or a response to such forces, can provide the necessary context. Cyclical forces can explain much of the climate variation we see through a wide variety of time scales.
One needs only to enter a car on a hot afternoon to recognize the greenhouse phenomenon. Solar energy, a broad-spectrum energy stream, enters a system through a transparent, friendly media, and contributes to an elevated temperature. That body, can re-radiate energy, but now as infrared radiation (IR). However, the media is not quite as friendly to IR radiation and less energy escapes. This retained energy contributes to the internal temperature.
Planet Earth behaves in an analogous fashion, with the friendly media being a very thin layer of natural greenhouse gases: H2O vapor, CO2, CH4 and NO2. This structure leads to a background warming of about 33°C, raising the planet's average temperature from around -18°C for a greenhouse gas free black body, to about +15°C today.
Enhanced global warming
The overall temperature change,DT, can be stated as shown in Equation 1 below. We will set aside the rather important questions as to what is the definition of the global average temperature and how it is measured. This DT can be written as:
DT = DTA + DTB + DTC (Equation 1)
where:DTA - anthropogenic warming; DTB - background warming; and DT2C - climate variation warming. Enhanced global warming, that is global warming beyond the large, basic warming that makes this planet habitable, can be defined as:
DTE = DTA + DTC (Equation 2)
While this equation looks almost trivial, the two tasks involved namely detection, (DTE > 0?), and attribution (percent of DTE allocated to DTA and percent to DTC) are incredibly difficult. Analogous equations can be written for other factors. For example, for sea surface temperature (SST), one could write:
DSST = DSSTA + DSSTC (Equation 3)
and for sea level rise (SLR) one could write:
DSLR = DSLRA + DSLRC (Equation 4)
Note, whatever allocation might ultimately be appropriate for Equation 2, a different allocation would apply for Equations 3 and 4, due to time lags and other factors.
Natural climate variation
The geologic record is starting to reveal the story of a massive flood that occurred 7.5 kilo-years before present (KYBP). This was the event where water broke through from the Mediterranean into the Black Sea (previously only a lake). This is an example of the magnitude of the forces and change involved in the world. This deluge was caused by the natural changes in sea and lake levels as the world emerged from the last ice age.
From 14 KYBP to 7.5 KYBP, the level of the oceans, including the Mediterranean Sea, increased by 75 meters. In contrast, over the same period, the Black Sea fell from 15 meters to 156 meters below present (mbp) after the ice sheet drainage from Europe and Russia dried up. Ultimately, the Mediterranean Sea obtained a sufficient level to break through the slender land barriers and convert the lake into the Black Sea. These sea level changes are huge in scope and consequence, and many other changes in nature are of similar size.
While the above equations talk about change, and clearly indicate the possibility of natural change, they do not speak at all to the magnitude or frequency of natural climate change. A perusal of climate history will show thousands of extreme events. Changes in weather and climate are perfectly natural.
Consider the number of natural cycles that contribute to our weather or climate. In a recent article, this author highlighted 12 cycles, part of a listing of 25 cycles. Another presenter has prepared a list of 75 cycles with climate involvement. Three major natural cycles that have a profound impact on our climate are:
- Glaciation cycles (100, 41 and 19-23 KYBP): Seven ice ages that have occurred over the past 600 KYs. This history is known to extend back at least 2 million years (MY). This is an incredible history showing the gradual, but chaotic growth of ice sheets followed by a relatively rapid decay. The ice accumulation would amount to 30-40 million cu km per cycle. And the amount of the planet's water molecules tied up in the cryosphere (ice) would vary over an approximate range of 2-6%. The driving forces behind these momentous changes are the Milankovitch cycles, the Earth's orbital eccentricity, tilt and wobble, with cycles of 100, 41 and 19 - 23 KYs respectively. This author calls these earth motion anomalies (EMAs). It is these EMAs that lead to changes in solar irradiation for specific locations and seasons and hence to the glaciation cycles.
- Glacier advances and retreats (250Y): Statements relating the retreat of mountain glaciers to global warming over-simplify the physics and geophysics behind this subject. This phenomenon has existed for at least 2 MYs, undoubtedly being influenced by the longer-term glaciation phenomenon. Over the last 10 KYs, about 14 glacial cycles have occurred, with perhaps four cycles over the past millennium. The latest cold cycle is the so-called Little Ice Age. These cycles are due to the cyclical behavior of the Sun, which produces a reduction in solar radiance at the bottom of this 250 (approximate) year cycle.
- Sunspot cycle (11 Y): Of the many solar cycles in nature, the best known is probably the sunspot cycle. This phenomenon reflects on the magnetic activity of the sun. More sunspots indicate more solar activity. This cycle has an average length of 11 years. The maximum for the current cycle (cycle 23) is forecast for May 2000 and is expected to be the highest ever. However, this forecasting is very embryonic with a recent reference noting that 28 prediction methods were used to arrive at a consensus.
A plot of sunspot activity versus SST first suggested a hint of a relationship between the number of sunspots and climate. A more recent study showed a most intriguing relationship, and possibly a correlation, between global temperature and the length of the sunspot cycle. This type of study now stretches back 240 years and picks up the eight major temperature trends that occurred over this period rather well.
The size of the cryosphere varied threefold over glaciation history. Hence, it is worthwhile to look at the structure of the hydrosphere and the hydrologic cycle. With no surprise, the oceans dominate, making up 96% of Earth's water. The cryosphere and underground water are the only other sectors that contribute significant volumes at roughly 3% and 1% respectively today. All other sectors - surface water, atmosphere and the biosphere add in only trace contributions.
The hydrological cycle quantifies the Earth's water budget between the sea, air, and land.
For the ocean and land sectors, the annual hydrological cycle fluxes are very much smaller than the reservoir size. This means that a huge number of years are required to turnover the reservoir. The exact reverse is true for the atmosphere, where many turnovers per year occur.
In any event, the accuracy of the numbers is poor, leading to a closure error for the overall cycle of a factor of two. In this context, it is interesting to note the status of the science in this section. This will be viewed from two extremes of the technological spectrum, namely from evaporation pans to cosmic rays.
- Evaporation paradox: As an example of the incomplete state of the science in this field, a problem with evaporation rates was recently reported. This addressed the so-called evaporation paradox, where the drop in measured evaporation, using evaporation pans at weather stations, was hard to reconcile with increases in global cloudiness and precipitation. The paradox is resolved by a more precise analysis of the evaporation pan settings and energy balances. The purpose here is not to dwell on the technology of evaporation or on it's measurement, but only to note that even this rather simple, fundamental property, one that has been used for over a century, needs re-analysis and interpretation.
- Cloud cover and solar couple:
Glaciation cycles, EMAs, glacial advances and retreats, sunspots, and cosmic rays are highly complex concepts. Science is just beginning to understand the astrophysics and atmospheric chemistry involved. The understanding of the interactions, feedbacks, amplifications, and time lags of these phenomena with our climate is surely even more embryonic.
The emergence of astrophysics as one of the key global warming sciences has been one of the major developments of the past decade. In particular, the role of the sun is starting to emerge as probably the principle force in our climate. Clearly, it plays the dominant role on a daily and seasonal time scale. It also plays a role on an 11-year and an approximate 250-year time scale and probably several other time scales.
It is hard to overstate the importance of the sun and astrophysics in this field. This area surely needs and deserves more research.
(Some material in this series is based on papers OTC 8689 and OTC 10773, presented at the 1998 and 1999 Offshore Technology Conference).
Bryant, E., "Climate process & change," Cambridge Press, 1997. Lamb, H. H., Climate, "History and the Modern World," Routledge, London, 1995
Westbrook, G., "After Kyoto, Science Still Probes Global Warming Cause," Oil & Gas Journal; January 19, 1998.
Westbrook, G., "Sea Levels and Global Warming," National Center for Policy Analysis, Brief #282, October 9, 1998.