While on the 1798 Napoleon expedition to Egypt, a French mathematician and physicist, Joseph Fourier, became fascinated with the behaviour of heat. Two decades later, he published the “Théorie Analytique de la Chaleur”, his seminal work on the mathematical laws describing the diffusion of heat in time and space. Today, Fourier’s mathematics is used to describe any kind of waveform, including light, heat and sound. In 1824, he applied this knowledge to understand how heat is transported between interplanetary spaces and the Earth; the results of this study suggested that the Earth was warmer than it should be given its distance from the Sun, leading him to postulate that “the temperature [of the Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, that in re-passing into the air when converted into non-luminous heat”.
In lay terms, sunlight passes through the atmosphere easily but when it is absorbed by the Earth’s surface and released as heat (infrared radiation), a different form of energy, the atmosphere acts as a barrier to its escape back into space. This atmospheric blanketing effect then results in elevated surface temperatures. Whilst this theory was oversimplified, it is often cited as the first reference to a natural “greenhouse effect”. We now know that the atmosphere does not behave in this way but it took a community of physicists to identify the atmospheric constituents and processes that lead to planetary warming.
Claude Pouillet and Samuel Langley’s contributions on measuring solar radiation and how it permeates through space were invaluable to this pursuit, but it was not until 1859 that John Tyndall showed that water vapour (H2O), carbon dioxide (CO2) and ozone (O3) were the atmospheric gases responsible for limiting heat escape. Although they were present in small quantities, they had especially effective heat-trapping potential, with water vapour having the greatest potency. The major atmospheric constituents—nitrogen (N2) and oxygen (O2)—in contrast, were almost transparent to radiation. He concluded, “without water vapour [in the atmosphere], the Earth’s surface would be held fast in the iron grip of frost”. That is, water vapour which makes up 0-4 per cent of the atmosphere (depending on location and season) was most essential in maintaining elevated global temperatures. These compounds that have the ability to absorb (and emit) solar radiation are now referred to as greenhouse gases (GHGs).
While experts had established the heat-trapping properties, it was not until 1896, and by Svante Arrhenius, that the planetary warming that GHGs could effect was quantified. Building on the works of Fourier and Tyndall, Arrhenius predicted that should the quantity of CO2 (then, carbonic acid) in the atmosphere be doubled, terrestrial temperatures would rise by 5-6°C.  Consequently, the combustion of coal, a major source of CO2 and the primary fuel source of the time, could cause a change in climatic conditions. He, however, thought that this change in climatic conditions would take thousands of years to manifest and would be a positive for the environment by delaying the next ice age.
The foregoing discoveries underpin our current understanding of the greenhouse effect: incoming solar radiation warms the Earth’s surface, which subsequently radiates the energy back as heat. Some heat is selectively-absorbed by GHGs and re-emitted in all directions, including back to the Earth. The remaining radiation passes straight through and returns to space. This constant flux of energy between atmosphere and surface is the Earth’s temperature valve, and changes to the composition of either the atmosphere or the Earth’s surface would disrupt the natural processes of our environment, e.g. winds, ocean currents, precipitation, that are driven by temperature differences.
By the start of the 20th century, the second Industrial Revolution was underway. A burgeoning world population and emerging industries drove the search for new energy resources. Oil, which was being discovered in vast quantities around the world, was poised to fuel the revolution because of its ubiquity and versatility. Oil is less laborious to mine and easier to transport than coal and results in less particulate pollution. The by-products of oil also had a range of applications including lighting, marine transport and power generation.
As oil became the most valuable commodity in the world and fuel consumption rose, there was an increasing consensus that human activity from rising fossil fuel use was influencing climatic factors via the greenhouse effect. From 1958 to 1961, Charles Keeling developed a device for measuring atmospheric CO2 concentration in air samples at Mauna Loa, Hawaii. The data resulting from the work showed rising atmospheric CO2 levels thus strengthening the existing evidence for climate change. Mauna Loa has since been the site of one of the foremost atmospheric research facilities.
Also during the second Industrial Revolution, a community of scientists concerned with quantifying climate dynamics using models was emerging. Climate models are mathematical representations of the processes that occur within the climate. Until the 1950s, these models were simple, using a few basic equations to describe one aspect of the system (e.g. oceans, the atmosphere, carbon cycle, etc.) in isolation. In the 1970s, atmosphere and ocean models were “coupled” to understand the exchange of heat and material (e.g. freshwater) between them.
From the 1980s, the focus shifted to integrating hitherto isolated models into a single model to account for the feedback mechanisms between different aspects of the climate system. Today, Earth System Models (ESMs) that incorporate the atmosphere, biosphere (vegetative cover and marine ecosystems), cryosphere (snow and ice cover) and biogeochemical (aerosols, sulphate, carbon) cycles have been developed. These models are continuously being improved and have become the tools with which we assess the risks of anthropogenic activity on the climate. Moreover, integrated climate and economic models, known as Integrated Assessment Models (IAMs), have become default tools for forecasting the “future” and assessing the physical, economic and social implications of implementing certain technologies and policies.
After 200 years of scientific discovery, our understanding of the Earth’s climate system is thus: it comprises the atmosphere, land surface (including snow and ice cover) and the oceans. Solar radiation is distributed between these bodies, which constantly absorb and re-emit it. This constant exchange of energy underpins the driving forces of the climate—winds, ocean currents, and radiation itself—and leaves the climate in a state of constant flux. The interactions between climatic factors and local factors such as latitude, topography and proximity to waters, determine local conditions.
The complex and chaotic nature of the climate system creates oscillations, which, until the latter half of the 20th century, were thought to be temporary perturbations from a natural equilibrium i.e. frequently changing weather patterns were thought to be a consequence of these perturbations that aggregates into a long-term stable climate. External forcings—events outside the atmosphere that influence the climate system—such as volcanic eruptions also influence the climate by shading the planet with ash or dust, thereby resulting in a temporary cooling which can last up to years.
The greatest exogenous disruptor to the climate system, however, is the enduring romance between humans and fossil fuels, which sparked at the dawn of the Industrial Revolution. Fossil fuels have since become the backbone of the modern economy. Their energies have brought economic growth and increased prosperity, but the resulting emissions have ushered in a period of unprecedented global warming. Air bubbles in Antarctic ice cores have given us insight into historical atmospheric CO2 levels and have confirmed that the current levels are far from an excursion from an equilibrium but are an upwards deviation at a rate unseen since the beginning of the Anthropocene.
The effects of this deviation will vary by region, but the bulk of scientific literature suggests that frequent extreme weather events (droughts, storms, hurricanes, etc.), changing precipitation patterns and agricultural yields, and temperature increases, are to be expected. Some effects such as rising sea levels and shrinking ice cover are already evident. Climate change now presents the greatest challenge to humanity. Restoring atmospheric GHG concentration to levels present before the Industrial Revolution is considered impossible in the near-term (the next 100 years), so global efforts have been geared to mitigation of its catastrophic effects and adaptation to a changing environment.
By the late 1980s, the climate change debate was becoming increasingly political. The wave of environmentalism that swept across the West had, hitherto, been focused on local pollution from oil spills, chemical use or nuclear wastes. This traction was partly owed to Selma Carson’s 1962 book Silent Spring, which brought public attention to the toxic effects of extensive pesticide use and the misinformation campaign by chemical companies on the safety of their products. The resulting movement highlighted the shortfalls of the United States government in securing and improving the state of its environment and human health, and led to the creation of the US Environmental Protection Agency.
Around the same time, a new class of compounds, chlorofluorocarbons (CFCs) had been demonstrated to be resistant to decomposition in the lower atmosphere (troposphere). There are two mechanisms through which compounds are removed from the atmosphere: deposition and reaction. Deposition involves the dissolution of compounds in water vapour so they can be removed as precipitation. Reaction is the breaking down of compounds by chemically reacting them with an oxidising agent. In the troposphere, these agents are mostly hydroxyl or nitrate radicals.
Chlorofluorocarbons, however, are insoluble in water and, unlike organic compounds that can be completely be removed by radicals in weeks, they react slowly and can have a residence time of up to 200 years. The chemical resistance of CFCs allows them to mix with the atmosphere till they reach the stratosphere—the layer containing ozone, which prevents harmful ultraviolet (UV) rays from reaching the Earth. There, CFCs are photodecomposed by intense UV radiation and release chlorine and fluorine radicals, which react with ozone, deplete the layer (commonly referred to as a “hole”,) and, increasingly, allow harmful UV rays to reach the earth’s surface.
Before this discovery, CFCs were being used in fire extinguishers, refrigeration systems, solvents, etc. An international treaty, The Montreal Protocol on Substances that Deplete the Ozone Layer (1987), recognised the consequences of unchecked ozone depletion and called for a global commitment to phase out CFCs and similar compounds. 197 countries have since ratified the agreement and remarkable progress has been made, with NASA reporting that the historical depletion of stratospheric ozone could be completely reversed by 2060. While not explicitly about climate change, Montreal marked the beginning of an era of international cooperation to address environmental issues.
In the same year, the United Nations World Commission on Environment and Development (WCED) published ‘Our Common Future’ (often called the Brundtland Report, after the chair of the commission) was the first to highlight that environmental degradation presents limits to economic growth, and called for the consideration of the environment in international development efforts. The report also produced the definition of sustainable development most widely used today: “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.
In 1988, the Intergovernmental Panel on Climate Change (IPCC) was established to “assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation”. The IPCC second assessment report (AR2) concluded that the “balance of evidence suggests a discernible human influence on global climate”. This brought widespread acceptance of climate change by the international community and soon after, in 1992, the United Nations Framework Convention on Climate Change (UNFCCC) was signed with the objective of “stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”. The UNFCCC came into force in 1994 after ratification by 50 states. Currently, all member states of the UN are party to the convention.
In AR2, the global warming potential (GWP) metric was used to evaluate the effectiveness of GHGs in causing planetary warming. CO2 was taken to have a GWP of 1 i.e. it was the benchmark to which other gases were compared. In a 20-year timeframe, water vapour and methane (CH4) were estimated to have GWPs of 45 and 62, respectively, much more potent than CO2. However, the abundance of CO2 emissions post-Industrial Revolution made it the greatest driver of warming and, hence, the area of greatest concern. The IPCC used this knowledge to design the methodology for taking greenhouse gas inventories that would be used to assess the progress of forthcoming climate agreements.
The Kyoto Protocol (1997) mandated 37 industrialised countries, including the European Community, to reduce or slowdown the growth of their GHG emissions. 100 developing countries, including India and China, were excluded from the treaty but could voluntarily comply. The US Congress, however, did not ratify the agreement, citing that it placed the burden of emissions reductions on industrialised countries and hence would disadvantage the US economy. The Byrd-Hagel Resolution (1997) stated that the US could not be a signatory to any such agreement “unless the protocol or other agreement also mandates new specific scheduled commitments to limit or reduce greenhouse gas emissions for Developing Country Parties within the same compliance period”. The resolution was passed with a 95-0 vote by the senate and President Bush withdrew the US from the protocol in 2001.
The lack of US participation and the rising levels of GHG emissions that have since occurred have left Kyoto as a failure. Kyoto owes some of this failure to its division of countries into Annex I parties (industrialised or near-industrialised economies) and ‘others’. This has been a source of tension and debate as to who should bear the burden of addressing climate change during international negotiations: should the burden be shared equitably so historically-responsible countries bear the brunt of solution-finding or should maximal efforts be made by all parties involved?
The 2015 Paris Agreement was lauded for overcoming these political barriers and having all countries commit to keeping global temperature rise “well below 2°C” by the end of the century. Syria was the last country to sign the accord in November 2017. Although President Trump has threatened to pull the US out of the treaty, imminent withdrawal is prevented by Article 28. The earliest permitted date for withdrawal is November 4, 2020 (one day after the next US presidential election). Withdrawal is also possible by leaving the UNFCCC, but this voids all previous treaties that the US has been party to under the convention. Ultimately, the Trump White House has signalled its intent to wait till 2020. Whether the threats to unravel the Paris Agreement are just strongman tactics from POTUS or a concerted effort to redefine the terms agreed is yet to be seen.
The Paris Accord owes its success to the manner in which commitments by individual parties were defined. Rather than dictating what should be done (which some have viewed as a breach of sovereignty in the past), the UNFCCC allowed parties to define their course of action to help achieve the 2°C target—their intended nationally determined contributions (INDCs). The accord is not without its negatives, however. While achieving a global consensus on an issue is admirable, this was made possible by the vagueness of the agreed terms and the non-legally-binding nature of the agreement. There are no repercussions should a country fail to comply with its set targets, except, perhaps, a loss of political goodwill.
The long-term nature of the target, which was negotiated for by less ‘climate-centric’ countries, fails to create the needed sense of urgency for immediate action. There is also no consensus on what is needed to achieve the 2°C target, but the recent IPCC Fifth Assessment Report (AR5) has helped to fill that knowledge gap. In AR5, the IPCC compiled scientific literature from IAMs to understand the socioeconomic pathways that lead to a range of global temperature rise. In summary, to have a 66 per cent probability of meeting the Paris target, a transformation of the global energy system, changes in global agricultural practices and consumer behaviour, and policies that increasingly value the social cost of GHG emissions must be implemented at a rate and scale hitherto unseen. An excellent study by Rogelj et al. showed that should all countries achieve their INDCs, global temperature would rise by 2.6-3.1°C. INDCs are, therefore, insufficient and must be strengthened if Paris is to be a feasible target.
In several decades, the decarbonization debate has shifted from why to how and that, in itself, is laudable progress. Climate change is now part of the mainstream political discourse, though action to address it has lagged. Our dependence on fossil fuels has been unchanged in the last three decades (they have supplied 86 per cent of global energy consumption since the 1980s). Resistance to pro-climate policies from fossil fuel-dependent industries with immense political lobbying power is in part responsible for this but, more so, the lack of economically-viable alternatives to fossil fuels.
In recent years, an unprecedented decline in the costs of renewable energy technologies harnessing the sun and wind has spurred a green revolution. The global installed capacity of renewables is growing exponentially, but they still only satisfied 2.8 per cent of the world’s energy consumption in 2016. The variability of their supply (weather is uncertain) and large costs of energy storage needed as back-up, however, still limit their widespread adoption on the required scale. Gas is set to replace coal as the preferred fuel for power generation. While gas produces significantly less GHG emissions, it is, nonetheless, a fossil fuel and long-term use should only be allowed if power plant emissions are abated using carbon capture and storage (CCS), a technology that stores the emissions from power plants deep underground.
Although CCS has been demonstrated as safe and effective, costs remain restrictive and favourable policies to incentivize investment in the technology and deployment are lacking. The increasing electrification of road transport is set to wean the transport sector off oil but, again, this is not enough. The International Energy Agency (IEA), in their 2018 World Energy Outlook, has shown that the rise of electric vehicles will only reduce projected oil demand by 2 per cent. BP’s Chief Economist, Spencer Dale, echoed the same sentiments at a lecture in Imperial College London in December 2017. BP predicts that a doubling of the forecast of electric vehicle use produces only a marginal change in oil demand because the decarbonization solutions for the drivers of demand—marine transport, heavy good vehicles (HGVs), aviation and petrochemicals—are yet to be found.
The foregoing may paint a gloomy picture but many hurdles have been cleared. Unsubsidised renewables will be competitive in the electricity system from the mid-2020s with solar power becoming the cheapest form of new electricity in 58 countries. Battery costs continue to plummet and present a potential long-term solution to the intermittency of renewable energy technologies. Credits for utilizing or storing carbon, such as the recent extension of the 45Q tax credit in the US, will encourage investment in CCS.
Carbon pricing and emissions trading schemes are undergoing reform to make them more effective in driving low-carbon innovation and investment; these schemes now cover a quarter of global GHG emissions. Fuel and energy efficiency policies are curbing energy demand growth. Mass production of electric vehicles for passenger and good transport is afoot in the motor industry, with sales projections constantly being revised upwards as cars become more affordable and charging infrastructure is rolled out in many cities. All these developments will create positive feedback and will drive further investment in low-carbon technologies that will strengthen decarbonisation efforts. Uncertainties around what paths to take remain but the destination is clear, and that is a decarbonized world⎈
 BBC Radio 4, Joseph Fourier, 2010.
 J. Fourier, Théorie Analytique de la Chaleur, Paris: Firmin Didot, 1822.
 J. Fourier, “Mémoire sur les Températures du Globe Terrestre et des Espaces Planétaires,” Mémoires d l’Académie Royale des Sciences de l’Institute de France, vol. VII, pp. 570-604, 1827.
 P. A. Kidwell, “Prelude to solar energy: Pouillet, Herschel, Forbes and the solar constant,” Annals of Science, vol. 38, pp. 457-476, 1981.
 C. Pouillet, Mémoire sur La Chaleur Solaire: sur les pouvoirs et absorbants de l’air atmosphérique et sur la température de l’espace, Paris: Bachelier, 1838.
 J. Tyndall, “On the Absorption and Radiation of Heat by Gases and Vapours, and On the Physical Connexion of Radiation, Absorption and Conduction,” in Contributions to Molecular Physics in the Domain of Radiant Heat, London, Longman, Green and Co, 1872, pp. 7-66.
 S. Arrhenius, “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground,” The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, vol. 41, pp. 237-276, April 1896.
 J. M. Andino and F. S. Rowland, “Chlorofluorocarbons (CFCs) are heavier than air, so how do scientists suppose that these chemicals reach the altitude of the ozone layer to adversely affect it?,” Scientific American, [Online]. Available: https://www.scientificamerican.com/article/chlorofluorocarbons-cfcs/ . [Accessed 1 May 2018].
 Radicals (also known as free radicals) are atoms, molecules or ions that have at least one unpaired electron in their outermost atomic orbit. Stable compounds have all electrons paired. Free radicals are highly reactive with one another, or other atoms, to gain an electron(s) and achieve this stability.
 United Nations, Montreal Protocol on Substances that Deplete the Ozone layer, Montreal: Secretariat for The Vienna Convention for the Protection of the Ozone Layer & The Montreal Protocol on Substances that Deplete the Ozone Layer, 1987.
 United Nations World Commission on Environment and Development, Our Common Future, Oxford: Oxford University Press, 1987.
 Intergovernmental Panel on Climate Change, Principles Governing IPCC Work, Vienna: IPCC, 1998.
 IPCC, Climate Change 1995: A report of the Intergovernmental Panel on Climate Change, Geneva: IPCC, 1996.
 United Nations, United Nations Framework Convention on Climate Change, Geneva, 1992.
 105th Congress (1997-1998), S.Res.98 – A resolution expressing the sense of the Senate regarding the conditions for the United States becoming a signatory to any international agreement on greenhouse gas emissions under the United Nations Framework Convention on Climate Change, Washington D.C., USA, 1997.
 United Nations, Paris Agreement, Paris: United Nations, 2015.
 IPCC, Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, United Kingdom and New York, USA: Cambridge University Press, 2014.
 J. Rogelj, M. den Elzen, N. Höhne, T. Fransen, H. Fekete, H. Winkler, R. Schaeffer, F. Sha, K. Riahi and M. Meinshausen, “Paris Agreement climate proposals need a boost to keep warming well below 2°C,” Nature, vol. 534, pp. 631-639, 2016.
 R. Boyd, J. C. Turner and B. Ward, “Intended nationally determined contributions: what are the implications for greenhouse gas emissions in 2030?,” ESRC Centre for Climate Change Economics and Policy; Grantham Research Institute on Climate Change and the Environment, London, 2015.
 British Petroleum Company, “Statistical Review of the World Energy,” British Petroleum Co., London, 2016.
 See also: M. Liebrich, “Bloomberg New Energy Finance,” 14 December 2016. [Online]. Available: https://www.bloomberg.com/news/articles/2016-12-15/world-energy-hits-a-turning-point-solar-that-s-cheaper-than-wind.
 Institute for Climate Economics (I4CE), “Global Carbon Account 2018,” I4CE, Paris, 2018.