ACS Climate Science Toolkit
Please take a moment to read the background and rationale for the ACS Climate Science Toolkit in the About section.
The goal of the Toolkit is to provide ACS members with a deeper understanding of climate science and to assist them in speaking about climate science with various audiences.
The ACS Climate Science Toolkit is presented in a modular format you can enter at different points, depending on your background in the various scientific concepts. Links throughout allow navigation between interrelated sections and explanations.
To provide possible starting points, the list of FAQs captures central questions that commonly arise in climate science discussions. Each question is linked to a brief answer that also includes links to the ACS Climate Science Toolkit sections that will provide more thorough explanations. Please pursue these topics in any order that interests you or that draws on your particular scientific background.
- What is the evidence that Earth is warming?
- What is the atmospheric greenhouse effect?
- What are the properties of greenhouse gases? What are the Earth’s greenhouse gases?
- How are a warming Earth and increased greenhouse gases related?
- Have human activities contributed to Earth’s greenhouse gases? How do we know that human activities increase greenhouse gases?
- What is the mechanism of the atmospheric greenhouse effect? What is radiative forcing?
- Where is the Earth’s carbon?
- What are feedbacks in the climate system? What is climate sensitivity?
- How are oceans affected by the Earth’s energy imbalance? How are oceans affected by increasing atmospheric CO2 levels?
- What role does chemistry play in climate science?
- What is the ACS position on climate science?
Long-term records of global warming indicators are evidence for warming. The average surface temperature of the Earth has increased by almost one degree Celsius in the past century. Earth’s ice is melting—most glaciers are melting and retreating, Arctic Ocean ice is disappearing, and the Greenland and Antarctic ice sheets are decreasing. Sea level is rising due to expansion of the warming oceans and added melt water from land ice.
The atmospheric greenhouse effect is a natural phenomenon that has kept the Earth warm enough for life as we know it for billions of years. The energy balance between absorbed incoming solar radiation and outgoing infrared radiation from a planet determines the average planetary temperature. Planets with an atmosphere that contains infrared absorbing gases are warmer than they would be, based on their distance from the sun, if they did not have this atmosphere. This is called the atmospheric greenhouse effect.
What is so special about a greenhouse gas that distinguishes it from other atmospheric gases? The atmospheric greenhouse effect requires gases that absorb radiation in the thermal infrared region of the spectrum, about 5 to 50 μm (2000 to 200 cm–1). Gases whose molecular dipole moment changes when the molecules vibrate meet this criterion. Heteronuclear diatomic molecules and all gases whose molecules have three or more atoms are greenhouse gases
3-B. What are the Earth’s greenhouse gases?
With the exception of N2, O2, and Ar, all atmospheric gases, whether natural or human-produced, are greenhouse gases. The largest contributor to greenhouse gas warming is water vapor. The second largest is CO2, which gets the most attention, because it is increasing and is the Earth’s thermostatic control. Contributions from other gases—CH4, N2O, tropospheric O3, and synthetic halogenated gases—are smaller but significant for their global warming potential.
There is a causal relationship, based on the mechanism of the greenhouse effect, between increased greenhouse gases and a warming Earth. The mechanism of the atmospheric greenhouse effect is well established both experimentally and theoretically. The mechanism predicts that the result of increasing non-condensable greenhouse gas concentrations is an increase in radiative forcing and, hence, an energy imbalance that adds extra energy to the planet, which results in warming.
There has been an increase in the concentrations of greenhouse gases during the past two centuries as well as the addition of new synthetic compounds. There are both natural and human contributions to most atmospheric greenhouse gases. Human activities have increased the concentrations of CO2, CH4, and N2O to levels unprecedented in at least the past million years. In addition, synthetic halogenated refrigerants and solvents have been added, as these came into widespread use starting in the 1950s.
5-B. How do we know that human activities increase greenhouse gases?
Human activities and not some natural variation in the Earth’s climate system are the cause of the increasing concentrations of greenhouse gases, particularly CO2. The increase in greenhouse gases has coincided with the great increase in fossil fuel burning associated with the Industrial Revolution and modern use of fossil fuel energy for transportation. The ratios of carbon isotopes in fossil fuels and hence CO2 from fossil fuel burning are different from those in the CO2 present in the atmosphere before the Industrial Revolution.
The analogy of the atmospheric greenhouse effect to a gardener’s greenhouse is inaccurate; the limits of the analogy need to be clear and the very different mechanisms understood. Planetary energy balance is maintained if outgoing thermal infrared radiation energy from the top of the atmosphere equals the absorbed incoming solar radiation energy. Planetary atmospheres that contain infrared-absorbing gases continuously absorb and reradiate the thermal infrared radiation emitted by the planetary surface. Atmospheres generally become colder with altitude, so energy balance is maintained by emission from the cooler atmosphere, while the surface temperature remains warmer.
6-B. What is radiative forcing?
Any factor that causes an imbalance between incoming solar and outgoing thermal infrared radiation is a forcing factor requiring the climate system to adjust to a new balance with the Earth either on average warmer or cooler than before. Variations in the Earth’s orbit and axial tilt or the sun’s output are forcing factors changing incoming solar energy. Changes in the Earth’s albedo—ice to water, emission of aerosol particulates, and the extent and nature of clouds, for example—alter the amount of the incoming radiation that is reflected away without warming the surface. Changes in non-condensable greenhouse gas concentrations and some effect of clouds and aerosol particles are forcing factors changing the outgoing energy flux.
On human timescales, the organic carbon in the biosphere and inorganic carbon, as CO2 in the atmosphere and oceans, are of greatest interest in climate science. However, more than 99% of the carbon on Earth is bound up in carbonate minerals (rocks). Over millions of years, this carbon is cycled back into our familiar, more rapidly interchanging reservoirs. Weathering, part of the carbon cycle, is speeded up by increased atmospheric CO2, but is too slow to have a significant effect on the climate.
The many interconnected parts of the climate system, atmosphere and oceans, for example, work synergistically to affect one another. Feedback occurs when the reaction of some part of the climate system to a change in a climate variable alters the amount (or even direction) of the variable change. A significant feedback in the Earth’s climate system is the increase in atmospheric water vapor as the Earth warms, because increased water vapor then further increases greenhouse warming.
8-B. What is climate sensitivity?
Future climate changes depend on how much the Earth will warm in response to added greenhouse gases, its climate sensitivity. Fundamentally, climate sensitivity is the change in average planetary temperature required to return to energy balance after a change in forcing has occurred. Much climate science attention has focused on forcing by greenhouse gases, especially CO2. Thus, temperature changes quoted for climate sensitivity are often based on calculations for the temperature rise that would result from a doubling of the CO2 concentration (or its equivalent including all non-condensable greenhouse gases) from the pre-industrial level.
The upper 700 meters of the oceans have warmed by about 0.2 °C since the middle of the 20th century, with deeper layers warming as well. This warming in the large volume, high heat capacity oceans has absorbed at least 90% of the energy imbalance caused by the increased greenhouse effect. Thermal expansion of ocean volume has contributed substantially to observed sea level rise during this period.
9-B. How are oceans affected by increasing atmospheric CO2 levels?
Airborne and dissolved CO2 are continuously exchanged between the atmosphere and the oceans. The increased atmospheric CO2 concentration increases the amount of dissolved CO2, which reacts with the carbonate species, HCO3–, CO32–, CaCO3, etc., to lower the ocean pH (ocean acidification). Changes in oceanic biology in a more acidic ocean are inevitable, but, at present, unpredictable.
Chemistry has a vital role in scientific understanding of atmospheric processes involving molecules’ interactions with each other and radiative energy. Further, the potential effects on the ocean are a matter of chemical reactions that affect the acid/base equilibrium of a very complex mixture over a range of temperatures. Thus climate science is underpinned by fundamental chemical concepts and any efforts geared to mitigating or adapting to the effects of climate change will invariably require deep chemical understanding.
The ACS is composed of many types of chemists including those involved with climate science research. Further, its members craft policy positions on issues of importance to the chemical enterprise and its practitioners. Among its over two dozen policy statements is one on Climate Change, taking the Position “Reviews the science and recommends action on reducing greenhouse gases as well as climate change adaptation strategies. Encourages continued research and funding into the effects of climate change, while also emphasizing the importance of educating the public on the issue.”