1.3: Why and How Is Climate Changing?

You have so far learned how certain pollutant gases behave like a blanket, trapping heat and causing global warming. In this section, we will document the evidence that these gases are changing our climate and do a deep dive into why and how the climate is changing.

Distinguishing between weather and climate

Weather is what is happening at any given time or on a short time scale of a few weeks or less. For example, there may be rain today, sunshine tomorrow, and a storm a few days later. These kinds of dayto-day short-term changes are what we call weather. Climate describes conditions over a longer term. For example, in many regions winter is colder and drier than summer. Summer might bring monsoons to some regions. These are descriptions of climate, which is essentially a longer-term average of weather. The greenhouse effect causes warming and other changes to climate on time scales of seasons or longer. A warmer climate in turn leads to other changes, such as extreme weather events (heat waves, droughts, extreme storm events). It’s in this context that we talk about climate change.

Distinguishing between global warming and climate change

Figure 1.3.1 Human activities that lead to climate pollution. Top left image by Alan Kiniry from Pexels. Top right image reproduced from Pixabay. Bottom left image by V. Ramanathan. Bottom right image reproduced from Pixabay.

Until about two decades ago, scientists used to refer to the increase in temperature due to increases in CO₂ as global warming. However, this term does not describe all of the impacts that go along with warming, such as extreme weather and rising sea levels. Moreover, in the 1990s and early 2000s, “global warming” became a politicized phrase and issue, particularly in the United States. Scientists have responded by avoiding the phrase global warming and replacing it with the phrase climate change. Both terms are used in this text because global warming and climate change are distinct processes. By definition, global warming refers specifically to the warming effect of anthropogenic gases on the planet. This global warming in turn leads to broader climate change, which includes changes in winds, storms, rainfall, and humidity.

Why is the planet warming?

As briefly described in the earlier sections, the Earth has been warming since the 1850s. The warming has not been constant or steady, however. As we will see, the evidence indicates that most of this warming is caused by human activities that release pollutant greenhouse gases into the atmosphere, thickening the natural greenhouse blanket. The gases began increasing in the 1850s, but the Great Acceleration in consumption that began in the 1950s (Section 1.1 and Figure 1.1.2) contributed to a steeper increase in the concentration of many gases during the last half of the twentieth century.

The four images in Figure 1.3.1 reveal the interconnectedness of the climate change problem. The woman cooking with firewood (that was my grandmother’s kitchen in south India) could lose her source of food because of changes in climate, such as droughts, caused by CO₂ emitted for the most part in developed countries. Likewise, the smoke coming from that woman’s kitchen in south India—as well as from cars in the US and power plants in China—could melt glaciers thousands of kilometers away. It is imperative to keep in mind that pointing fingers at each other will not solve the climate change problem. We are all in this together and together we must solve this problem.

We have already identified carbon dioxide as a major anthropogenic greenhouse gas. Carbon dioxide is a significant concern in part because of its long lifetime in the atmosphere. Roughly half the emitted CO₂ will be taken out of the atmosphere in less than a decade by the land biosphere (trees, plants, and soil) and by the ocean, but the remaining half will stay in the air for at least 100 years, and about 20% of the CO₂ will stay in the atmosphere for 1,000 years or more. You, your children, your grandchildren, and future generations yet to be born will still be inhaling the carbon dioxide emitted by your car today.

The impact of aerosols

One important point to note from Figure 1.3.1 is that the visible smoke and smog shown in the images is made up in part of tiny particles called aerosols; carbon dioxide and other greenhouse gases cannot be seen by the human eye. Most of these aerosols reflect sunlight and have a cooling effect, but black carbon aerosols (a major component of soot) absorb solar radiation entering the atmosphere and have a warming effect. This trapping of incoming solar radiation should not be confused with the trapping of outgoing infrared radiation emitted by the surface. Often black carbon is referred to as a greenhouse gas. This is wrong on two counts: black carbon is not a gas, and black carbon warms the climate by absorbing solar energy rather than infrared energy from the planet. Black carbon is mainly produced by incomplete combustion. Major anthropogenic sources include internal combustion engines in vehicles (particularly diesel-powered vehicles) and the burning of solid coal, firewood, crop residues, and animal dung (for heating and cooking).

Fertilizing agricultural fields and burning fossil fuels and biomass fuels (for example, wood) also contribute to other types of aerosol particles, such as sulfates, nitrates, and organics. Unlike black carbon, these other aerosol particles primarily reflect sunlight and have a cooling effect. Although some of this cooling is offset by black carbon’s warming, the net effect of all aerosols combined is one of cooling. This cooling has been estimated to offset about a third of the warming caused by anthropogenic greenhouse gases, but the net impact of human emissions still warms the planet.

Super pollutants

As of 2010, non-CO₂ pollutants (non-CO₂ greenhouse gases and black carbon) contribute about 45% of the total anthropogenic warming effect. These non-CO₂ greenhouse gases and black carbon particles are also called super pollutants. This is because, per molecule, their warming effects are much larger than that of CO₂. For example, methane is 25 times more potent than CO₂ at warming the planet; nitrous oxide is 300 times more potent; HFCs and CFCs are a few thousand to 10,000 times more potent; and black carbon is 2,000 times more potent (also Box 1.3.1). These non-CO₂ pollutants have powerful warming effects, but methane, ozone, HFCs, and black carbon are also called short-lived climate pollutants (SLCPs) because their lifetimes in the air range from less than a week (black carbon), to a month (ozone), to a decade or two (methane and HFCs), compared with the century to millennial time scales of CO₂. These relatively short atmospheric lifetimes will be an important factor when we begin to look at climate solutions.

Warming trends

Box 1.3.1 Global Warming Potential of Greenhouse Gases

Each greenhouse gas has a different capacity to trap heat in the atmosphere. One way we can measure this is through global warming potential (GWP), which compares the heat-trapping effect of a gas to the effect of an equal mass of carbon dioxide.

Different gases stay in the atmosphere for different time periods; scientists call the time a particular gas remains its lifetime. Since the warming effect of a gas depends in part on how long it stays in the atmosphere, global warming potential must be defined for a specific time period, usually 20 years or 100 years.

The table below lists the 100-year global warming potential (GWP100) for three of the most important greenhouse gases. For example, the 100-year GWP of methane is given as 30 (with a range of 28 to 36). This means that if we were to emit equal masses of methane and carbon dioxide into the atmosphere at the same time, the methane would trap 30 times as much heat energy as the carbon dioxide over a period of 100 years.

Greenhouse gas	Chemical Formula	Lifetime in the Atmosphere (years)	GWP (100 years)
Methane	\(\ce{CH4}\)	12	30
Nitrous Oxide	\(\ce{N2O}\)	114	298
HFC-134a*	\(\ce{CH2FCF3}\)	14	1,430

* HFC-134a is a commonly used refrigerant and is given as an example of a hydrofluorocarbon (HFC). There are dozens of different HFCs in use, with GWP values ranging from a few hundred to several thousand.

We can use global warming potentials to define “equivalent emissions” in terms of CO₂. Scientists call this the CO₂ equivalent, typically written as “CO₂e” or “CO₂eq.” For example, since methane has a GWP of 25, the release of 1 ton of methane would have a warming effect comparable to 25 tons of CO₂. This might be described as the addition of 25 tons of CO₂e. When looking at greenhouse gas emission numbers, it’s important to note whether they’re expressed in tons CO₂ or tons CO₂e.

You may have noticed that carbon dioxide is not included in the table; its GWP is 1 by definition. Also, as we will see, carbon dioxide is removed from the atmosphere by a variety of different processes, so it’s not possible to define a single lifetime for CO₂.

Also notice that two important greenhouse gases, water vapor and ozone, are not included in the table. That’s because their lifetimes in the atmosphere are extremely short, only a few days or weeks, so it’s not meaningful to define a 100-year global warming potential for them.

Sources: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T. F., et al. (eds.)]. Lifetimes, Radiative Efficiencies and Metric Values, Table 8.A.1 Cambridge University Press, New York, NY.; UNEP. 2012. The Emissions Gap Report 2012. United Nations Environment Programme, Nairobi, Kenya.

Figure 1.3.2 Changes in global averaged land and sea surface temperatures since 1880, relative to 1951–1980 average. Adapted from NASA/GISS.

Signs of warming can be seen on the land and sea surface as well as in the atmosphere and the deeper oceans. The globally averaged surface temperature shown in Figure 1.3.2 reveals a persistent warming that began in 1900 and continues until the present (2018), with some ups and downs. Most of the 1°C warming experienced since the beginning of the twentieth century happened after the Great Acceleration began in the 1950s.

A similar pattern is observed in the ocean to a depth of at least 700 meters. The warming can be seen over the whole globe with very few exceptions. Most every region has experienced the warming, but it is not uniform. For example, the land surfaces have warmed more than the sea surface. This is expected since the land surface has less thermal inertia than the sea and hence warms more rapidly than the ocean. The Northern Hemisphere has warmed more than the Southern Hemisphere, again largely because of the ocean’s influence: the spatial extent of the ocean is not as great in the Northern Hemisphere. The northern polar regions have warmed twice as much as the global average: more than 2°C compared with the global average of 1°C.

If we keep adding climate pollutants at the present rate, global temperatures will continue to increase to more than 2°C by 2050, and to a catastrophic 3°C to 7°C by end of this century.

How do we know the warming is due to human activities?

Figure 1.3.3 Observed temperatures compared with those from models using only natural factors and with those from models using both natural and anthropogenic factors. Reproduced from IPCC.

A large amount of evidence and many lines of evidence-based reasoning have led scientists to conclude unequivocally that the warming is caused by the increase in the thickness of the greenhouse blanket of CO₂, methane, CFCs, ozone, and nitrous oxide. There are two primary grounds for this conclusion:

1. Natural changes are much too small to produce the observed warming. There are three main ways that natural changes can contribute to climate change. First, changes in processes within the sun can cause variations in incoming solar energy. However, incoming solar energy has been regularly monitored by satellites since the late 1970s, and the observed variations in incoming solar energy are about a factor of 10 lower than the 3 W/m2 increase caused by anthropogenic thickening of the greenhouse gas blanket. Even more significantly, changes in solar output over the last couple of decades have been in the opposite direction. That is, the sun’s energy output has decreased slightly, which would tend to cause cooling, not warming.

A second natural factor that can affect climate is variation in the Earth’s orbit around the sun. These orbital changes play a significant role in climate changes on time scales of 10,000 years or more (for example, the cycles between glacial and interglacial periods), but they have negligible effects on time scales of a century or so. They are simply too slow to be responsible for the warming observed over the past few decades.

The third natural factor that can cause climate changes is volcanic eruptions. Volcanoes put out sulfur gases that get converted into reflective aerosol particles in the atmosphere. By reflecting solar energy back into space, these particles cool the climate. Volcano-induced cooling is real but lasts for less than 5 years. The change in reflected solar radiation due to volcanoes and the resulting temperature changes have been measured from both surface instruments and satellites. For example, the eruption of Mount Pinatubo in the Philippines in 1991 produced a measurable drop in global temperatures for at least 2 years. As the sulfate particles are gradually removed from the atmosphere, temperatures tend to return to previous levels. Although volcanoes do emit carbon dioxide, these emissions are less than 1% of human-generated CO2. Scientists have concluded that apart from temporary cooling, volcanoes have had very little effect on the rapid warming trend observed since the 1980s.

2. Models can simulate the observed warming only if they include human activities. The most sophisticated climate models to date account for both natural variations and the human-caused increase in greenhouse gases. Model runs that include only natural variations show year-to-year fluctuations in temperatures, but they completely fail to reproduce the current warming trend. Only when models include the anthropogenic thickening of the greenhouse blanket do they reproduce the observed warming of the planet. We can see this in Figure 1.3.3. The black lines represent observations, the blue regions represent the range of predicted temperatures from models that include only natural factors, and the pink regions indicate the range of projections from models that include both natural and anthropogenic factors. The observed warming is far outside the range of projections that include only natural factors, but it is well within the range of projections that include anthropogenic factors as well. This leads climate scientists to conclude that anthropogenic changes are the dominant factor in recent warming.

Why trust the models?

This leads us to a question: Why should we trust the models? After all, they are just computer calculations. How do we know they accurately reflect the real world? Scientists trust the models in attributing the observed warming to human activities because, in general, the model projections are consistent with the observed changes. Models have successfully predicted many changes that were later observed, a few of which are listed below:

• In 1980, models were used to predict that CO2-induced warming would be detected by the year 2000. Indeed, in 2001 the comprehensive report written by over 1,000 scientists for the IPCC was the first to formally conclude that there was a discernible warming in the observed records.
• Models predicted that warming induced by greenhouse gases would penetrate to the deeper oceans. Scientists have deployed thousands of underwater probes in every major ocean basin, and their measurements show that warming temperatures have penetrated to at least 700 meters below the surface.
• Models predicted that the greenhouse-gas-induced warming would extend to the entire lower atmosphere (from the surface up to above 12 kilometers). This has been confirmed by balloon and satellite data.

The predictions suggested that a warmer atmosphere would become more humid and that the increase in water vapor would in turn amplify the warming because water vapor is a powerful greenhouse gas. Humidity data collected by weather balloons and microwave instruments on satellites confirmed that water vapor has increased with the increase in temperature since the 1980s.

In the late 1960s, a Russian meteorologist predicted that as the planet warmed, sea ice and snow would retreat, making the surface less reflective and exposing the darker ocean below to solar energy. This reduced reflectivity would increase the solar energy absorbed by the Arctic Ocean, amplifying the warming. Indeed, satellite data have shown that the Arctic sea ice has retreated significantly since the late 1970s, followed by an increase in solar energy absorption by the Arctic Ocean and amplified warming. The Arctic region has warmed by almost 2.5°C, compared with the global average warming of 1°C.

But models are tested not just by their ability to successfully forecast changes in climate that are later observed. A typical test for modern climate models is their ability to reproduce past climate observations, such as the temperature record for the twentieth century. This process is called hindcasting. The ability of models to pass such tests increases scientists’ confidence that they include the factors necessary to determine the causes of observed climate change, as well as to project changes likely to occur in the future.

Based on the results from models and other observations and analyses, the most recent report of the US Global Change Research Program, composed of 13 federal departments and agencies, concluded in 2017 that “it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”

Projecting future warming: climate feedbacks

As we have seen, we have a good scientific understanding of temperature increases over the past century. Warming is driven primarily by increases in concentrations of greenhouse gases. This warming has been partially offset by the net cooling effect of aerosols.

Box 1.3.2 Climate Change: Is the Science Settled?

Media coverage of climate issues sometimes gives the impression that there is significant scientific debate about climate change. In reality, the scientific community largely agrees about climate change—both the fact that it is occurring and why it is occurring. This understanding of the mechanics of climate change is based on fundamental physics and well-established scientific principles. We address some of the most common questions about the scientific consensus on climate change here.

What fraction of the warming is due to human activities?

My best estimate: 80% or more. How did I arrive at such a number? The science tells us that the variations in natural climate forcing (that is, solar and volcanic activities) are too small to account for the observed warming trends and at times contrary to them. Further, both pedagogical and complex climate models are able to simulate the observed warming magnitude (0.9°C to 1°C) only if they include the observed buildup of greenhouse gases since 1900. See Box 1.3.3 for details of these calculations.

So, is the science settled?

The answer depends on what aspect of climate change science you ask about. Some of the most important questions have been answered with a high degree of confidence, as summarized in Table 1.3.1.

What aspect of the science is not settled?

Predictions of future warming are less certain. In the first place, we do not know how much climate pollution humans will emit over the coming decades. Even for a particular emissions scenario, however, climate models give a wide range of estimates. Some of the major reasons for this range include varying assessments of factors such as aerosols, cloud feedbacks, and other feedbacks due to the response of soils and plants to warming temperatures.

With sufficient warming, there is also the possibility of abrupt and irreversible changes if global temperatures cross “tipping points” that can push the climate into new states. Examples of tipping points include significant methane releases from melting permafrost or large-scale changes in ocean circulation. Unfortunately, the temperature thresholds for these tipping points are not well understood.

These feedbacks and dynamic processes mean that we must present any conclusion regarding the Earth’s future warming as a probable range rather than a single value.

Table 1.3.1 Summary of scientific consensus

Question	Reply
Is the atmosphere getting more polluted?	Yes
Are the greenhouse gases CO2, methane, and others increasing?	Yes
Are the increases due to human activities?	Yes
Is the climate warming?	Yes
Is the warming in part due to human activities?	Yes
What fraction of the warming is due to human activities?	50%–90%
What human activities are responsible for the warming?	Increase in CO2, other greenhouse gases, and black carbon particles due to human activities
Can we make precise predictions of future temperatures?	No. We can only provide probabilistic values.

Past and future warming is governed by climate feedbacks, which happen when the climate system responds to temperature increases in ways that can either amplify or moderate warming. Three of the most important feedbacks we need to consider in relation to climate change in the twentieth and twenty-first centuries are the following:

1. Water vapor feedback: We have already discussed this feedback earlier in the chapter. When the temperature of the atmosphere increases, it holds more water vapor. Since water vapor is a greenhouse gas, this feedback acts to amplify warming, resulting in temperature changes that are roughly twice as large as would be expected from the increase in greenhouse gases alone.
2. Ice-albedo feedback: As described in the previous section, increasing temperatures reduce snow and sea ice cover, which decreases albedo and amplifies warming. This feedback has its strongest effect in the Arctic, which is why this region has warmed substantially more than the global average.
3. Cloud feedbacks: Clouds can affect temperatures in two different ways. Clouds reflect sunlight, which tends to cool the Earth. However, the liquid water or ice crystals in clouds also trap infrared radiation, causing warming. It turns out that low, thick clouds have a net cooling effect, while high cirrus clouds have a net warming effect. Thus, the overall feedback from clouds depends on whether a warmer world would have more low, thick clouds or more high cirrus clouds. Including cloud effects in computer models is difficult because of their relatively small size and complex formation processes. The current scientific consensus is that, overall, cloud feedbacks are likely to have a small amplifying effect on warming. However, cloud feedbacks continue to be one of the largest sources of uncertainty in computer projections of future temperatures.

Box 1.3.3 Can Climate Science Account for the Observed Warming Trends?

Certainly—let me walk you through a little bit of math. We’ve already mentioned that scientists measure incoming solar radiation and outgoing heat in units of watts per square meter (W/m²). The physics behind the heat-trapping effect of greenhouse gases is well understood, so we can calculate the imbalance they create in outgoing versus incoming radiation. Scientists call this imbalance greenhouse gas forcing. For the amount of greenhouse gases in the atmosphere in 2005, we find that the forcing is about 3 W/m².

How much warming would that 3 W/m² forcing be expected to cause? To calculate this, we divide the greenhouse gas forcing by a number called the climate feedback parameter. Our best estimate of this number is 1.3 W/ (m² °C), read as “1.3 watts per square meter per degree Celsius.” This means that a forcing of 1.3 W/m² would be expected to raise the global temperature by 1°C. Thus we are able to derive the theoretical warming that we should have seen from greenhouse gases alone by dividing 3 W/m² by 1.3 W/(m² °C), resulting in an expected warming of 2.3°C. However, we have only observed 1°C. Where is this difference coming from?

First, not all of the warming appears at the Earth’s surface; approximately 0.5°C is stored by the oceans. Also, greenhouse gas forcing is not the whole story. About 0.7°C of the expected warming is reversed by aerosol cooling, and 0.2°C is reversed by changes in surface albedo, mainly due to clearing of forests for agriculture and grazing. When we subtract out warming that was stored by the oceans or reversed (2.3 − 0.5 − 0.7 − 0.2), we arrive at 0.9°C expected warming for the surface (Table 1.3.2). This is a good match for the 1°C warming that has been observed.

What about natural factors, such as changes in the energy radiated by the sun, volcanic eruptions, or natural variability due to heat exchanges between the oceans, atmosphere, and land? These factors have been examined carefully, and the conclusion is that they could cause the global temperature to vary up or down by as much as 0.2°C. In short, natural factors alone are far too small to account for the observed 1°C warming. We can only account for the observed warming by including the effects of anthropogenic greenhouse gas emissions.

Source: Myhre, G., et al. 2013. Anthropogenic and natural radiative forcing. In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T. F., et al. (eds.)]. Cambridge University Press, New York, NY. https://www.ipcc.ch/site/assets/ uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf.

Table 1.3.2 Theoretical warming breakdown

Factor	Warming
Greenhouse gas forcing (2005)	3 Wm^−2
Climate feedback parameter	1.3 Wm^−2 °C^−1
Theoretical warming we should have seen with just greenhouse gas forcing (= greenhouse gas forcing divided by the climate feedback parameter = 3/1.3)	2.3°C
Observed warming	1°C
Ocean heat storage. This is the heat energy stored in the ocean, and it will be released as surface warming in a few decades.	−0.5°C
Masking by aerosol cooling	−0.7°C
Surface albedo changes	−0.2°C