2. KNOWLEDGE AND RISK: WHAT WE KNOW ABOUT WHAT WE DO AND DON’T KNOW
Note. It is proposed that the final chapter 2 will comprise a short section on oil and fossil fuel reserves and the risk characteristics associated with oil depletion; a substantive section on climate science; and a section on the analytics of cost-benefit and risk under uncertainty.
Current material on oil reserves has been included in Chapter 4. The draft chapter here presents the material on climate science. Economists and risk scientists may also wish to look at the draft “Chapter 2 material input on cost-benefit and risk under uncertainty”, on the specialist review material part of the book website; key elements of this will be presented in the first section of the review meeting at Imperial College.
1. What We Do Know.
One thing we certainly do know is that the Earth is warming. To call it ‘global warming’ wouldn’t be inappropriate. What’s more, the science behind it is really quite simple. The greenhouse effect has been known about for nearly 250 years and has been linked to ‘greenhouse gases’ for well over a century. These gases are warming our planet. The large number of ways in which this warming impacts our intricate climate system, however, is a far more complicated story. This range of impacts we call climate change. Although there are still uncertainties, change is happening and is gaining momentum. The most obvious and easily measured changes are temperature increases, but changes in weather patterns and other natural systems are also occurring and look set to continue to do so. But let’s go back to the basics.
The burning of fossil fuels is changing the atmosphere. The inevitable increase of carbon in the Earth system is the most direct impact of human activity on our planet, and evidence shows rapid rises in atmospheric greenhouse gases in recent years. Records from air bubbles trapped in ice show that atmospheric CO2 levels, which have been steadily fluctuating between 180 and 280 ppm for the last 800,000 years (Dieter Lüthi, 2008), have shot to a current 387 ppm since the industrial revolution began, 150 years ago (Aresta, 2010). This current level, and the rate of this increase, is backed up by direct air measurements (Keeling, 2009), and it unparalleled in the entire ice core record (Weitzman, 2009).
What else do we know? Well, greenhouse gases keep the Earth warm. Like the glass in a greenhouse, the layer of gases in the upper atmosphere lets sunlight pass through, but traps other forms of heat – such as the heat re-radiated from the surface of the Earth after it’s been warmed by the sun’s radiation (figure 2.1). This greenhouse effect is responsible for keeping our planet at a habitable temperature; without carbon dioxide (CO2) amongst other gases in the atmosphere, the sun’s heat would be reflected straight back into space, and the surface temperatures of Earth would be on average 33°C colder (Le Treut, 2007). The planet’s water bodies would be permanently frozen, and life probably wouldn’t have evolved. Hence, it is the huge amount of CO2 released by volcanic activity in Earth’s early history that enabled the planet to make the shift from bare, rocky and uninhabitable to one where life could exist.
Figure 2.1. The greenhouse effect. Source: http://www.co2crc.com.au
The greenhouse effect was first noted by H.B. de Saussure in the 1760s, who was surprised that the natural phenomena of the “hot room or carriage”, where glass lets in heat in the form of sunlight but traps it in other forms, hadn’t yet been formally studied. Citing his work in 1824, Joseph Fourier related the effect to the Earth’s atmosphere, and the subsequent warming of the Earth’s surface (Fourier, 1824), and from this the idea of the greenhouse effect was developed.
Of the so-called ‘greenhouse gases’ (GHGs), carbon dioxide (CO2) is most often mentioned, but methane (CH4), nitrous oxide (NO), and water vapour (H20) also play a large part. They are all released during human activities – primarily through the burning of fossil fuels, but also large scale land-use change, deforestation and certain industrial processes – and add to the gas layer already present in our atmosphere. Human activities have been thickening this blanket of gases since the industrial revolution, and It’s been predicted for a long time that this could lead to the warming of the Earth. But is there evidence for this?
Well, yes; and it’s not easy to ignore. A general trend of increases in surface, oceanic and atmospheric temperatures are among the most clear and obvious examples, each adding to the picture of a planet heating up slowly but surely. Most strikingly, the ten hottest years since records began have all occurred since 1997 (based on land and marine surface temperatures from the Meteorological Office (MetOffice, 2009)). The last time the annual global mean temperatures were below the 1901-2000 mean was in 1976; since then, they’ve consistently been warmer than that average (NOAA, 2008); see fig. 2.2). Additionally, the decadal scales strengthen the pattern. The 2000s decade (2000–2009) was warmer than the 1990s, which in turn were warmer than the 1980s and all earlier decades (MetOffice, 2009). Although there are recent reports of a recent slowdown in the observed warming between 2000-2009 (Easterling., 2009), these are apparently not inconsistent with expected internal variability, and are therefore consistent with the 0.2°C/decade observed trend (Knight J, 2009). has been It’s clear; surface temperatures are increasing.
Fig. 2. 2. Surface temperatures are increasing.
Global mean temperatures compared to the 1901-2000 average. Source: NOAA, 2008; http://www.epa.gov/climatechange/science/recenttc_triad.html.
Worryingly, so are the temperatures of the oceans themselves – by far the planet’s largest heat reservoir, estimated to accumulate over 80% of the climate system’s excess heat (Le Treut, 2007) (Levitus, 2005a). There has been a general trend of increases in the heat content of the entire oceans over the last half century (see Figure 2.3), despite a significant amount of spatial and temporal variability (S. Levitus, 2009). Although almost all the warming occurrs in the upper 700m of the oceans, the measurable warming of such a vast amount of water indicates a significant increase in the climate system’s external radiative forcings.
Fig. 2. 3. The Oceans are Warming.
Yearly ocean heat content (1022J) for the 0–700 m layer from Levitus et al. [2009] (solid) and Levitus et al. [2005a] (dashed). Each yearly estimate is plotted at the midpoint of the year. Reference period is 1957–1990. Source: Levitus et al. 2009 (S. Levitus, 2009).
Finally, certain parts of the Earth’s atmosphere are warming while other parts are cooling, as is expected from anthropogenic greenhouse gas-driven changes. The troposphere, the part of the atmosphere closest to the Earth’s surface and where all greenhouse gases are found, has warmed, whilst the part directly above, the stratosphere, has cooled. Evidence comes from radiosconde observations with near-global coverage date from 1958, and satellite-based temperature measurements which began in 1979. All data sets show that the troposphere has warmed, at between 0.12 to 20°C per decade since 1979 (Qiang Fu, 2004) (Karl, 2006) (Le Treut, 2007). Distribution of tropospheric warming can be seen in Figure 2.4. Meanwhile the same datasets show that the stratosphere, the part of the atmosphere directly above the troposphere, has undergone considerable cooling over the same period (Ramaswamy, 2001) (Karl, 2006) a conspicuous fingerprint of the greenhouse effect – but more on that later.
Figure 2.4. The Troposphere is Warming.
Patterns of linear global temperature trends from 1979 to 2005 estimated for the troposphere from the surface to about 10 km altitude, from satellite records. Grey areas indicate incomplete data. Source: Modified from IPCC AR4 WG1 CH3 (2007).
So the direct evidence that the Earth is warming comprises three distinct types of temperature record, each from different sources and largely long-term datasets:
· Surface temperatures
· Ocean heat content
· Troposphere temperatures
Just as predicted from pumping out vast quantities of greenhouse gases, the concentrations of these gases in the atmosphere have increased rapidly, enhancing the greenhouse effect, and warming the Earth. Furthermore, these changes in our climate system have been heavily buffered (Miller, 2008), thanks to carbon sinks such as the oceans and forests absorbing much of the carbon (Sabine et al., 2004) and resulting heat (Le Treut, 2007). This has two worrying implications: if we were to stop emitting gases completely, the inertia of the system might mean that changes continue to occur for many years afterwards; and, we’re in the risky position of being at the mercy of these carbon sinks – should the capacity or function of any of them start to reduce, the impacts of emissions would likely rapidly accelerate.
2. What else is changing?
The knowledge that the world is warming, in ways consistent with expectations, is reinforced by numerous other observed changes, which also start to give us a sense of the areas in which accumulating heat might impact the world around us.
In geological terms, the presence of sizeable ice sheets at each of Earth’s poles mean that we can be described as being in an icehouse world. The Earth has fluctuated between icehouse states, where permanent ice exists, and greenhouse states with no permanent ice, throughout the last few hundred million years (e.g. (Kenneth G. Miller, 1991)). These changes are driven by a number of interacting factors, including variation in the Earth’s orbit around the sun, ocean circulation, albedo effects, etc., which combine to set about changes to the Earth’s state. These changes between greenhouse and icehouse states and involving the melting of icecaps happen over timescales of tens- or hundreds of millions of years, although smaller fluctuations can happen over periods of tens of thousands of years (e.g. (Thomas, 2008)). Global change over shorter timescales than this is virtually unheard of in the geological record, the exceptions involving tipping points leading to localised rapid change.
Yet currently sea ice on both poles is declining, and at an appreciable rate. Arctic sea-ice has been decreasing since records began in the late 1970s, losing an area of ice roughly equal to the size of France every decade since then (MetOffice, 2009). Recent years have witnessed huge declines, with the years 2007, 2008 and 2009 being respectively the first, second and third lowest recorded extents of sea ice since records began, although these still fall within the expected long-term trend of decline (MetOffice, 2009).
In addition to sea-ice, evidence of the retreat of glaciers and ice caps from both direct and satellite observations provide among the most visible and definite indications of changes in our climate system. The most recent report by The World Glacier Monitoring Service (WGMS) which reviews changes in over 700 glaciers from 27 different countries and regions worldwide, describes a ‘strong acceleration of glacier melting’. They state that rates of mass losses have more than doubled compared to 1980-2000 rates in their 30 ‘reference’ glaciers , monitored almost continuously since 1976 (Haeberli, 2008).
Even the most persistent glaciers, having survived large climate fluctuations in the past, are currently experiencing rapid declines. The northern ice fields of the glacier on Mount Kilimanjaro, dating back at least 11,700 years, survived a widespread drought 4,200 years ago that lasted around 300 years. Yet it has lost 85% of the ice cover ice cover present in 1912, and 26% of the cover present in 2000 is now gone (L. G. Thompson et al., 2009). This hints at the severity of current conditions.
The melt water has to go somewhere and the inevitable consequence of sea-ice and glacier melt is an associated sea-level rise. The Antarctic and Greenland ice sheets are each vast, between them holding enough freshwater to cause sea-levels to rise by 73 metres if they were to melt completely (MetOffice, 2009). This is unlikely to happen in a hurry, but significant increases in global mean sea level have been documented (see Figure 2.5).
Figure 2.5.Sea levels are rising.
Variation in sea level from the 1980-1999 mean. Before 1870, global measurements are not available and the grey shading shows the uncertainty in the estimated long-term rate of sea level change. The red line shows the global mean sea level from tide gauges. The green line shows global mean sea level observed from satellite altimetry. Source: adapted from IPCC 2007 AR4 WG1 CH5.
It appears that the sea-level has risen by increasingly greater amounts since the early 1900s, and ice melt has contributed as much as 0.8 ± 0.2 mm per year to sea level rise for the period 1993-2003 (Dyurgerov, 2005) (Hegerl, 2007) (Domingues, 2008). To put these figures into some sort of perspective, this would require the melting around 290 giga-tonnes of ice each year (Wouters, 2008).This sea level rise due to melting has been attributed to the heat from extra radiative forcing due to increased greenhouse gases. This however isn’t the only effect the rising temperatures have on sea-level. The warming causes the oceans to undergo thermal expansion, which adds to the sea-level increases. The sea surface temperature increases from 1880 until the present time are estimated to have produced a sea level rise of around 35 mm through thermal expansion, with as much as 16 mm occurring between 1993 and 2003 (Marcelja, 2010), exhibiting a recent increase in expansion rate due to temperature increases (S. Levitus, 2009) (Fasullo, 2009) (Domingues, 2008) (Hegerl, 2007).
Additional observed changes worth noting are the changing precipitation patterns. The global trend shows a general increase in drying areas (see Figure 2.6a), but there is a marked variation in the pattern over different regions, as shown in Figure 2.6b. The PDSI (Palmer Drought Severity Index) measures the cumulative deficit in land moisture compared to the historical average. Red and orange indicates areas that are drier than average, and blue and green areas are wetter than average. As shown, certain areas have become significantly wetter, for example northern Europe, parts of North and South America and northern and central Asia, whilst there has been a drying trend over many land areas, including much of Africa and especially the Sahel, the Mediterranean, southern Eurasia and Canada and Alaska. In fact, the number of areas classified as ‘very dry’ (with a PDSI of less than -3.0) have more than doubled since the 1970s.