Climate Change: The Evidence

In this lesson, we’ll explore the scientific evidence behind climate change, focusing on three key elements:

• The evidence of increases in greenhouse gases (GHGs) in the atmosphere.
• Resulting rising global temperatures.
• The harms associated with these changes.

Increasing GHG Prevalence: What's The Evidence?

As we covered in our last lesson, greenhouse gases (GHGs) are gases in the Earth’s atmosphere that trap heat. While some GHGs occur naturally and are necessary to maintain life-supporting temperatures on Earth, human activities have dramatically increased their concentration, intensifying the greenhouse effect.

The evidence for increasing greenhouse gas (GHG) levels is extensive and comes from multiple lines of scientific research. Researchers have built a comprehensive understanding of historical and current GHG concentrations through both ancient records and modern technology.

1. Ice Cores: Earth's Climate History

One of the most compelling sources of evidence for rising GHG levels comes from ice cores. Ice cores are long cylinders of ice drilled from the polar ice sheets in Antarctica and Greenland. These cores contain tiny bubbles of air trapped over hundreds of thousands of years. By analysing these trapped air bubbles, scientists can determine the concentrations of carbon dioxide (CO₂), methane (CH₄), and other GHGs from different time periods.

How Ice Cores Work

Each layer of ice corresponds to a specific time period, similar to the rings of a tree. As snow falls and compacts over time, it traps small air bubbles within the layers of ice.

By drilling deep into the ice, scientists can extract samples from different depths, each representing a different era in Earth’s climate history. The chemical analysis of the gases in these air bubbles reveals the concentration of GHGs at specific points in time.

What Ice Cores Reveal‍

Ice cores have provided a climate record going back as far as 800,000 years. They show that, for most of this period, CO₂ levels fluctuated between 180 to 280 parts per million (ppm).

However, starting in the mid-18th century, coinciding with the Industrial Revolution, CO₂ levels began to increase rapidly. Today, they have surpassed 420 ppm, a level not seen for at least the past 800,000 years.

Similar trends are observed for methane and nitrous oxide, with sharp increases beginning in the 18th and 19th centuries.

This dramatic increase in GHG levels is a clear indicator of human impact, as it coincides directly with the large-scale burning of fossil fuels, widespread deforestation, and changes in agricultural practices.

2. Atmospheric Measurements: Real-Time Monitoring

While ice cores offer a window into Earth’s climate past, modern atmospheric measurements provide real-time data on GHG concentrations. One of the most significant and long-standing records of atmospheric CO₂ is the Keeling Curve.

The Keeling Curve‍

Named after Dr. Charles David Keeling, who began monitoring atmospheric CO₂ at the Mauna Loa Observatory in Hawaii in 1958, the Keeling Curve is the longest continuous record of atmospheric CO₂. This observatory, located on the slopes of the Mauna Loa volcano, was chosen for its remote location, far from urban centres, to ensure accurate measurements.

The Keeling Curve reveals a consistent upward trend in CO₂ concentrations over the past six decades. When Dr. Keeling began his measurements, atmospheric CO₂ levels were around 315 ppm. By 2023, they had reached over 420 ppm, marking an increase of more than 100 ppm in just over 60 years.

This upward trend is punctuated by a seasonal fluctuation caused by the natural carbon cycle. During spring and summer in the Northern Hemisphere, plants absorb CO₂ through photosynthesis, causing levels to drop slightly. In autumn and winter, when plants die and decay, CO₂ levels rise again.

This natural cycle is superimposed on the long-term upward trend caused by human activities.

3. Satellite Observations: A New Perspective on GHG Emissions

In recent decades, satellite technology has revolutionised the way scientists monitor GHG levels. Earth-observing satellites provide a bird’s-eye view of the planet, allowing researchers to measure GHG concentrations across the entire globe with high precision.

How Satellites Measure GHGs‍

Satellites equipped with specialized sensors, such as the OCO-2 (Orbiting Carbon Observatory-2) and the Sentinel-5P, measure the amount of sunlight reflected off the Earth’s surface and absorbed by gases in the atmosphere.

These sensors can detect the specific wavelengths of light absorbed by CO₂, methane, and other GHGs, providing detailed maps of their concentrations.

What Satellites Reveal‍

Satellite data has provided invaluable insights into regional and global trends in GHG emissions. For instance, satellites have identified methane hotspots associated with oil and gas extraction, agriculture, and landfills. They have also helped track CO₂ plumes from large industrial centers, providing real-time data that can be used to enforce emission regulations.

For example, data from the Copernicus Sentinel-5P satellite revealed that an explosion in a natural gas well in Ohio in February 2018 released more than 50,000 tonnes of methane into the atmosphere. The blowout leaked more of this potent greenhouse gas in 20 days than the majority of many European nations do in a year from their oil and gas industries.

‍More GHGs, More Heat?

So there are more GHGs in the atmosphere - but is this really causing warming?

Numerous data points and observational studies now point to a clear correlation between increasing GHG concentrations and abnormal temperature rises. This chapter explores the key pieces of evidence that demonstrate how human-induced GHG emissions are causing global warming and other related climatic changes.

1. Long-Term Temperature Trends

Temperature records from around the world reveal that the Earth’s average surface temperature has risen by approximately 1.2°C (2.2°F) since the late 19th century. This increase is far beyond what would be expected from natural climate variability alone.

Instrumental Temperature Records‍

Since the late 1800s, scientists have been measuring global temperatures using thermometers stationed across land and oceans. These records show a consistent warming trend, particularly from the 1970s onwards, which aligns with the rapid increase in CO₂ concentrations. The years 2016 and 2020 were the hottest years on record, and the past decade was the warmest on record according to the World Meteorological Organization (WMO).

Paleoclimate Data‍

For a longer-term view, scientists analyse natural climate records from sources like tree rings, ice cores, and ocean sediments. These paleoclimate data provide insights into temperatures over thousands of years. They show that today’s warming is happening at a rate and magnitude that is unprecedented in at least the last 2,000 years. The temperature rise of the past century stands in stark contrast to previous periods of natural warming and cooling.

2. Attribution Studies

Attribution studies use climate models to separate human-induced warming from natural factors. These models take into account various influences on the Earth’s climate, including volcanic activity, solar radiation changes, and GHG emissions. By running models with and without human influences, scientists can isolate the effects of human activities.

Natural vs. Anthropogenic Influences‍

When models include only natural factors such as volcanic eruptions and solar variations, they show little to no long-term warming over the past century.

However, when anthropogenic GHG emissions are factored in, the models accurately replicate the observed warming trends. This robust finding strongly supports the conclusion that human activities are the primary driver of the observed warming.

Fingerprint Studies‍

These studies examine specific patterns of warming in the atmosphere. For example, if the warming were due to an increase in solar radiation, we would expect to see warming throughout all layers of the atmosphere.

However, observations show that the troposphere (the lower atmosphere) is warming while the stratosphere (the upper atmosphere) is cooling—consistent with increased GHG concentrations trapping heat in the lower atmosphere.

3. Accelerating Warming of the Oceans

The oceans play a crucial role in regulating the Earth’s climate by absorbing and storing heat. In fact, the oceans have absorbed over 90% of the excess heat trapped by GHGs since the 1970s. This massive heat absorption has led to a significant rise in ocean temperatures.

Ocean Heat Content‍

Measurements of ocean heat content show a steady increase in the amount of heat stored in the upper layers of the ocean. This warming is not confined to surface waters; it extends to deeper layers as well. The observed increase in ocean heat content is consistent with the amount of heat trapped by rising GHG levels.

Sea Surface Temperatures‍

Rising sea surface temperatures have been documented worldwide, with particularly sharp increases in the North Atlantic and Pacific Oceans. Warmer oceans contribute to more intense hurricanes and typhoons, and they also accelerate the melting of sea ice, leading to higher sea levels.

4. Melting of Polar Ice and Glaciers

The warming of the Earth’s atmosphere and oceans has led to widespread melting of ice in the polar regions and mountain glaciers. This phenomenon is one of the most visible indicators of global warming.

Arctic Sea Ice‍

The extent and thickness of Arctic sea ice have been decreasing at an alarming rate. The National Snow and Ice Data Center (NSIDC) reports that the Arctic sea ice minimum extent has declined by about 13% per decade since satellite records began in 1979. The Arctic is warming at more than twice the global average rate, a phenomenon known as Arctic amplification.

Greenland and Antarctic Ice Sheets

‍The Greenland ice sheet is losing ice at an accelerating pace. Satellite data shows that Greenland is losing approximately 280 billion tons of ice per year, while the Antarctic ice sheet is losing around 150 billion tons annually. These losses contribute significantly to rising sea levels, which are already impacting low-lying coastal regions.

Mountain Glaciers‍

Glaciers in mountain ranges around the world, including the Himalayas, Andes, and Alps, are shrinking. The retreat of these glaciers threatens freshwater supplies for millions of people who depend on glacial meltwater during dry seasons.

The Harms: Impacts of Climate Change

Rising temperatures are already having widespread impacts on the planet and human societies. These harms are projected to intensify as warming continues, affecting everything from weather patterns to biodiversity and public health.

1. Extreme Weather Events

One of the most visible consequences of global warming is the increase in extreme weather events, such as:

• Heatwaves: Hotter temperatures are making heatwaves more frequent and severe. For instance, Europe experienced unprecedented heat in 2019, with temperatures exceeding 40°C (104°F) in several countries. Heatwaves pose serious health risks, particularly to vulnerable populations like the elderly and those with preexisting conditions.
• Droughts: Warmer temperatures can exacerbate drought conditions, leading to water shortages and threatening agriculture. Regions like the American West and parts of Australia have experienced prolonged droughts, impacting crops, livestock, and water supplies.
• Storms and flooding: Warmer oceans provide more energy for storms, increasing the intensity of hurricanes, typhoons, and cyclones. Recent examples include Hurricane Maria (2017), which devastated Puerto Rico, and Typhoon Haiyan (2013), one of the most powerful storms on record. Alongside stronger storms, rising sea levels are leading to more frequent and severe coastal flooding, threatening cities like Miami and Jakarta.

2. Rising Sea Levels and Ocean Acidification

As the oceans warm and ice sheets melt, sea levels are rising. Over the past century, global sea levels have risen by about 20 centimetres (8 inches). The rate of sea-level rise has accelerated in recent decades, increasing from about 1.4 mm per year in the early 20th century to 3.3 mm per year in the past few decades.

3. Impacts on Ecosystems and Biodiversity

Climate change is disrupting ecosystems and putting many species at risk of extinction. Key impacts include:

• Habitat loss: As temperatures rise, many species are losing their habitats. For instance, Arctic species like the polar bear depend on sea ice, which is rapidly disappearing. Coral reefs, which support around 25% of marine species, are also under threat from ocean warming and acidification.
• Species migration: Many species are shifting their ranges in response to changing temperatures, moving toward the poles or to higher altitudes. However, some species cannot migrate fast enough or face barriers like urban areas and agriculture, leading to biodiversity loss.
• Ocean acidification: The oceans absorb about 30% of the CO2 emitted into the atmosphere. This excess CO2 reacts with seawater to form carbonic acid, leading to ocean acidification. This process harms marine life, particularly organisms like coral, shellfish, and plankton, which rely on calcium carbonate to build their skeletons and shells.

4. Impacts on Agriculture and Food Security

Climate change threatens global food security by affecting crop yields, water availability, and growing seasons. Key challenges include:

• Heat stress: Many staple crops, such as wheat, maize, and rice, are sensitive to temperature increases. Extreme heat can reduce crop yields and even cause crop failure.
• Changes in precipitation: Erratic rainfall patterns and prolonged droughts can disrupt agricultural production. For example, droughts in East Africa have contributed to food shortages and famine.
• Pests and diseases: Warmer temperatures are allowing agricultural pests and diseases to thrive in new regions, further threatening food production.

5. Human Health and Well-being

Climate change is also a major public health threat, with impacts including:

• Heat-related illnesses: More frequent heatwaves increase the risk of heat stroke, dehydration, and cardiovascular problems.
• Air quality: Higher temperatures can worsen air pollution by increasing the formation of ground-level ozone, which is harmful to respiratory health.
• Spread of diseases: Climate change is expanding the range of vector-borne diseases like malaria and dengue fever as warmer temperatures allow mosquitoes to thrive in previously cooler regions.

The Urgency of Action

The evidence linking rising GHG concentrations to global warming is undeniable, and the resulting harms are already affecting ecosystems, economies, and human health. While the impacts of climate change are complex and far-reaching, the core driver is clear: human activities, particularly the burning of fossil fuels, are causing the planet to warm.

By understanding the scientific evidence, we can advocate for and implement the necessary solutions to combat climate change and build a more sustainable future.