Summary

Several independent series of observations demonstrate that there has been about 1°C of warming since the start of the industrial revolution. We discussed that there is some variability in solar output, and that these variations may be recognized in records of past climate, but also that solar variability can only account for warming by 0.1°C to an unlikely maximum of 0.35°C since the end of the Little Ice Age. Based on energy balance considerations, we have found that our emissions of external carbon are the main culprit. In response to this disturbance of the energy balance, the climate simply has to change toward a warmer Pliocene- like state, even if we could manage to stabilize CO2 at its current level of about 400 ppm. From discussion of several slowly adjusting processes within the climate system, we now understand that it will take, from the beginning, several centuries to approximate the full Pliocene- like warming. But we are almost two centuries down the road, and warming to date already amounts to about 1°C. The slow components in the climate system will cause continuing warming by another 1°C or so. In other words, we are already committed to further warming, even if we managed to make the massive jump to a zero- emissions society from today and thus stabilize CO2 levels. The urgency of slashing back the current level of annual emissions (10 GtC) cannot be overstated. Every year of inaction brings us closer to the inevitability of a future climate that will exceed even the warm Pliocene state, with global temperatures at least 2 or 3°C higher than the pre- industrial level. If we allow ourselves to reach the Paris Climate Conference’s agreed maximum of 2°C warming by the year 2100, then the further commitment over coming centuries would take us toward 4°C, even if we achieved zero emissions by 2100. That is considerably warmer than during the Pliocene. We have seen that the consequences are grave. Progression toward a Pliocene- like climate state will be accompanied by continued migration of global and regional climate zones and by intensification of the evaporation and precipitation cycle, placing many areas at risk of increasing extremes, including aridity, flooding, and lethal heat.

Introduction

In 2015, the annual mean global atmospheric carbon dioxide (CO2) level surpassed 400 parts per million (ppm; Figure 1.1), and we know very well that this rise is caused by human activities (Figure 1.2). It was the first time in 3 million years that such a level had been reached. Crossing this level has caused widespread concern among climate scientists, and not least among those called pale climatologists, who work on natural climate variability in prehistoric times, before humans. Over the last few decades, researchers have been repeatedly raising the alarm that emissions of CO2, along with those of other greenhouse gases, are getting dangerously out of control and that urgent remedial action is needed. With the crossing of the 400 ppm threshold, this sense of urgency reached a climax: at the Conference of Parties 21 meeting in Paris—also known as COP21 or the 2015 Paris Climate Conference—broad interna¬tional political agreement was reached to limit global warming to a maximum of 2°C, and if at all possible 1.5° C, by the end of this century. If one calculates this through, this implies a commitment for society to operate on zero net carbon emissions well before 2050, along with development and large-scale application of methods for CO2 removal from the climate system. (Scientists focus on carbon (C) emissions when they discuss emissions because it helps in calculating CO2 changes produced by the processing of specific volumes/ masses of fossil fuel hydrocarbons.) Clearly, the challenge is enormous, especially given that even implementing all the pledges made since COP21 would still allow warming to reach about 3°C by 2100. But, regardless, the agreement was ground breaking. It was a marker of hope, optimism, and international motivation to tackle climate change. Moreover, there are concerns about the stated COP21 targets. First, the proposed 2°C or 1.5°C limits to avoid 2 “dangerous” climate impacts may sound good, but there is no specific scientific basis for picking these particular numbers. Second, the implied “end of this century” deadline is an arbitrary moment in time.

Causes Of Climate Change

The causes of natural climate variations, before human impacts, typically arose from one or more of the following: carbon- cycle changes, astronomical changes in the Earth- Sun configuration, large volcanic eruptions (especially plate- tectonics- related major volcanic episodes), asteroid impacts, or variations in the intensity of solar radiation output. Carbon-cycle changes may have acted on their own but were often also involved as a feedback in amplifying the climate responses to changes driven initially by the other mechanisms. In the following sections, we will look at each of these processes in turn. When we want to discuss the dominant changes in greenhouse gas concentrations, we focus mainly on CO2 and CH4 , of which CO2 is the dominant one on longer timescales because it exists in much higher concentrations and lasts much longer in the atmosphere than CH4. As mentioned before, we then commonly investigate things in terms of carbon (C) emissions and uptake because this allows us to relate variations directly to changes in the carbon cycle and how we humans are affecting it. The carbon cycle represents an intricate web of interactions that control carbon storage and exchange between the biosphere (life), hydrosphere (oceans, lakes, rivers), and lithosphere (rocks and sediments; Figure 4.1). We need to consider two critical terms when discussing it. The first is known as the reservoir volume. This stands for the volume of carbon held within each reservoir, such as land- plants and trees, the ocean, or carbonate rocks. The second term is known as flux, and it refers to the amount of carbon that is exchanged between two reservoirs in a year. Because the volumes of carbon that are involved are enormous, we commonly express them in gigatons (Gt). One gigaton is one billion (one thousand million) tons, where a ton is 1000 kg. Most frequently, this term will be used in the expression gigaton of Carbon, or GtC. There are several important reservoirs of carbon (Figure 4.1). The atmosphere holds an approximate volume of 750 GtC. The land-biosphere—living flora and fauna—comprises some 600 or 700 GtC of living material and more than 2000 GtC of dead material.

Past Climates: How We Get Our Data

On the work floor, research on past climates is known as paleoclimatology, and research on past oceans as paleoceanography. But they are very tightly related, and we shall discuss both combined under the one term of paleoclimatology. Within paleoclimatology, interests are spread over three fundamental fields. The first field is concerned with dating ancient evidence and is referred to as chronological studies. These studies are essential because all records of past climate change need to be dated as accurately as possible to ensure that we know when the studied climate changes occurred, how fast they were, and whether changes seen in various components of the climate system happened at the same time or at different times. The second field concerns observational studies, where the observations can be of different types. Some are direct measurements; for example, sunspot counts or temperature records. Some are historical, written accounts of anecdotal evidence, such as reports on the frequency of frozen rivers, floods, or droughts. Such records are very local and often subjective, so they are usually no good as primary evidence. But they can offer great support and validation to reconstructions from other tools. Besides direct and anecdotal data, we encounter the dominant type of evidence used in the discipline. These are the so- called proxy data, or proxies. Proxies are indirect measures that approximate (hence the name proxy) changes in important climate- system variables, such as temperature, CO2 concentrations, nutrient concentrations, and so on. This chapter outlines some of the most important proxies. The third field in paleoclimatology concerns modeling. It employs numerical models for climate system simulation and simpler classes of so-called box- models. Numerical climate models range from Earth System models that are relatively crude and can therefore be set to run simulations of many thousands of years, to very complex and refined coupled models that are computationally very greedy and thus give simulations of great detail but only over short intervals of time. Box- models are much simpler and faster to run, and they are most used in modeling of the carbon cycle or other geochemical properties.

Epilogue

There can be no doubt that the discovery and widespread use of fossil fuels has driven massive economical and societal developments. It started with coal. Although oil and gas became important thereafter, coal never really lost its position of prominence, although the tide may have turned in very recent years (Figure 1.2). The use of fossil fuels is familiar to virtually everybody on the planet, and a massive global corporate infrastructure exists to ensure its supply and distribution. In addition, petrochemical products are essential base materials for much of the manufacturing industry (e.g., plastics, synthetics, solvents, etc.). Virtually all of humanity has thus become thoroughly dependent on fossil fuels and other petrochemical products. We feel in our comfort zone with them. The very closely interwoven relationship between the modern way of life and fossil fuels and other petrochemicals, the emotive comfort- zone issue, and direct financial interests are key to the strong reactions that commonly follow whenever external carbon emissions are identified as an urgent problem. I hope that this book has provided you with a deeper understanding of the concepts and observations behind the concerns expressed by researchers. The role of CO2 in climate could perhaps be defined a little more sharply in the fine details, but the big lines are clear. And the big lines are all we need to appreciate where we stand today. Using just a few simple numbers about the potential consequences of the modern CO2 levels, with examples from the past, and equally simple numbers about the sheer quantities of carbon involved, everyone can get their heads around the scale of the problem. By similarly working out the scale of the required remedial measures in basic terms, we have seen that our most advisable course of action is to create a zero emissions society at the very soonest. Because there has been inaction for so long already, positive action on a truly global scale has now become a matter of the greatest urgency. This zero emission society is critical for stopping the unnaturally rapid rise in CO2 levels and stabilizing them to a level as close to today’s 400 ppm as we can.

Changes During The Industrial Age

Most of the 1°C temperature change since the start of the industrial revolution has occurred in the last six decades (Figure 1.1). The warming is evident in all independently monitored timeseries of global temperature. The general warming trend has been overprinted by variability on a lot of different timescales, largely because of internal (re-) distributions of heat within the atmosphere- ocean system. The world ocean, with an average depth of 3700 m, has more than 1000 times the heat capacity of the atmosphere. Even just the upper 700 m that are in effective exchange with the atmosphere have 200 times the heat capacity of the atmosphere. As a result, even a tiny fraction of a degree centigrade change in just the upper ocean represents an enormous amount of heat. This means two things: first, atmospheric temperature can be substantially affected by almost undetectable changes in the ocean; and second, ocean heat gain calculation requires very precise temperature measurements. Precise measurement series for the ocean only exist since about 1960. Let’s have a look at what atmospheric and oceanic heat gains tell us about the Earth’s energy balance since the industrial revolution. The roughly 1°C rise of Earth’s surface temperature during the indus-trial age, with more than two- thirds of it since about 1960, represents the “realized” response to forcing. Using standard values for global climate sensitivity to radiative forcing, we can determine that this 1° C warming corresponds to a component of climate forcing of roughly 1.1 to 1.3 W/m2. In contrast, the ocean is such a vast reservoir to heat up that it has not yet realized its full warming—ocean warming will therefore continue to develop over many decades to centuries even if we managed to “freeze” all radiative forcing agents at their current levels. Since 1960, the heat content of the upper 2000 m of the ocean has increased by roughly 27 x 1022 joules in about 55 years. This is an enormous number; namely 27 followed by 22 zeroes. For comparison, the most powerful nuclear detonation ever had a yield of about 22 x 1016 joules.

Mother Nature To The Rescue?

Now we come to the key issue. Many discussions about climate change turn to the well- known fact that (very) large CO2 fluctuations have happened in the geological past. This is then taken to imply that “we shouldn’t worry: nature has seen this all before, and will somehow clean up our external carbon emissions.” The veracity of this sentiment can be tested by considering the main mechanisms available in nature for extracting carbon from the atmosphere-ocean system. These are weathering, reforestation, and carbon burial in soils and sediments. In the next section, we look at the potential of these processes. Thereafter, we consider the case for human intervention, and potential ways forward. A first mechanism by which nature has dealt with past high- CO2 episodes is chemical weathering of rocks. In warmer and more humid climates, chemical weathering rates are increased, and this extracts CO2 from the atmosphere. However, CO2 removal through weathering at natural rates is an extremely slow process, which operates over hundreds of thousands to millions of years. Given time, there is no doubt that natural weathering will be capable of eventually removing the excess CO2, but this process is so slow that it offers no solace for the future, unless we are prepared to wait many hundreds of thousands of years. There may be some future in artificially increasing the weathering processes to remove anthropogenic carbon, but this is in its infancy—we will revisit this in sections 6.2 and 6.3. A second mechanism for carbon extraction from the atmosphere-ocean system concerns expansion of the biosphere, most notably through reforestation. We have discussed this before in terms of expansion and contraction of the biosphere during ice- age cycles. In today’s case, carbon extraction through biosphere expansion requires first that the industrial age’s trend of net deforestation is reversed. Interestingly, this actually may have happened at around 2003. Between 2003 and 2014, net global vegeta¬tion increased by about 4 GtC (i.e., at an average rate of about 0.4 GtC per year), due to a lucky combination of increased rainfall on the savannahs of Australia, Africa, and South America, regrowth of forests on abandoned farmland in Russia and former Soviet republics, and massive tree- planting projects in China.

ENERGY BALANCE OF CLIMATE

The Sun is the ultimate energy source for climate. The Sun radiates toward Earth at an almost constant intensity of about 1360 watts per square meter (W/ m2), as measured above the Earth ’s atmosphere. Most of this radiation takes place in the short ultra- violet and visible light wavelengths. We refer to it as incoming short- wave radiation (ISWR; the wavelengths are short because the Sun radiates at very high temperatures of about 5500°C). Earth is not a two- dimensional disk, but a 3- dimensional sphere. Its day- side faces the Sun and receives radiation, while its night- side is directed away from the Sun and does not receive solar radiation. As a result, the global average energy received from the Sun per square meter of Earth surface is the energy received by the day- side of Earth averaged over the surface area of the entire sphere. When we do the mathematics, this gives an average input of solar radiation into every square meter of Earth, at the top of the atmosphere, of 340 W/ m2 (Box 3.1). That is the value that things work out to when considering the ISWR from the Sun in a continuous and globally equally “smeared out” sense, and that is what matters when we are working out the balance between energy gained and lost by Earth (Box 3.2). Many people are puzzled by the fact that we talk only about energy from the Sun. They then especially wonder why we ignore heat input from the deep Earth, and in particular from volcanoes, which after all are very hot. But in spite of the spectacular shows of heat, steam, gases, and primordial mayhem that volcanoes put on display, they turn out to be almost negligible in terms of heat flow into the climate system. Compared with the global average solar energy gain of 340 W/ m2, recent assessments show that total heat outflow from the Earth’s interior is not even 0.09 W/ m2.