Climate Change Evidence and Explanation

The Reality of Climate Change

Climate change is unequivocally happening, and human activity is the primary cause. Multiple lines of evidence, from satellite measurements to on-the-ground observations, confirm that the planet is warming at an unprecedented rate. The scientific consensus on this matter is as strong as it is for well-established facts like the Earth being a planet or the existence of air. Leading scientific organizations worldwide, such as NASA, the IPCC, and the National Academies of Sciences, Engineering, and Medicine, agree on the reality and human causation of climate change. Despite this consensus, many still debate the reality of climate change, often fueled by misinformation.

Addressing Skepticism

Many people are aware of the environmental concerns related to climate change, such as the threat to polar bears, and may support initiatives like electric cars. However, they often encounter staunch opposition, with claims that climate change is a hoax perpetrated by scientists and other groups. These claims often originate from politically or economically motivated sources that seek to undermine climate science. Understanding the basis and flaws in these arguments is crucial for informed discussions.

This note serves as a reference to explain the evidence supporting climate change and its human cause. It aims to provide a clear, fact-based understanding of the issue, counteracting skepticism with scientific data and analysis.

The Greenhouse Effect

The sun is the primary source of warmth for the Earth. When sunlight reaches Earth, some of the sun's light is reflected by ice and clouds, while the rest is absorbed by land and water, which then re-emit this energy as heat in the form of infrared radiation. Some of this heat escapes into space, but a portion is trapped by the atmospheric greenhouse effect. This effect is caused by gases in the atmosphere, such as carbon dioxide, methane, and water vapor, which absorb and re-emit the infrared radiation, preventing it from escaping into space. The greenhouse effect is essential for life on Earth, maintaining a habitable temperature. However, human activities, particularly since the Industrial Revolution, have increased the concentration of carbon dioxide (CO2) by 40%, along with other greenhouse gases, intensifying the effect and leading to global warming.

Solar Activity vs. Greenhouse Gases

While the sun's output has varied throughout history, influencing Earth's climate, data shows that since the 1970s, when global temperatures have increased most rapidly, solar activity and temperature have moved in opposite directions. Measurements of solar irradiance from satellites show no net increase in solar energy reaching the Earth. If increased solar activity were the cause, both the upper and lower layers of the atmosphere would warm together. Instead, warming is only observed in the lower layers (troposphere), while the upper layers (stratosphere) are cooling, which corresponds to the accumulation of human-produced greenhouse gases like CO2. This pattern is a key piece of evidence distinguishing between solar and greenhouse gas forcing.

Carbon Dioxide Emissions

Since 1870, human activities, including the burning of fossil fuels (coal, oil, and natural gas), cement production, and changes in land use (deforestation), have added approximately 2,000 gigatons (two trillion tons) of CO2 into the atmosphere. Deforestation reduces the number of trees available to absorb CO2. About 40% of this CO2 has remained in the atmosphere, while the rest has been absorbed by oceans and land sinks. Analysis of gases trapped in ice cores reveals that current CO2 levels, exceeding 400 parts per million, are the highest they have been in nearly a million years—predating human existence. These levels have increased dramatically since the Industrial Revolution, driven primarily by human activities.

Rising Temperatures

The increase in atmospheric carbon dioxide is causing a rapid rise in global average temperatures. The Earth is currently warming about ten times faster than it did at the end of the last ice age. Climate models and historical data indicate a strong correlation between CO2 concentrations and global temperatures. The current warming trend is consistent with the predicted effects of increased greenhouse gas concentrations.

Evidence Linking CO2 Increase to Human Activity

The most compelling evidence linking the increase in CO2 to human activity comes from analyzing carbon isotopes in the atmosphere. Carbon isotopes are variants of carbon atoms with differing numbers of neutrons. Fossil fuels are derived from old plants, which preferentially use the lighter carbon-12 isotope over the heavier carbon-13. Therefore, they contain a higher ratio of carbon-12 to carbon-13 compared to the atmosphere.

When fossil fuels are burned, the proportion of carbon-12 in the atmosphere increases, which aligns with observations. This isotopic fingerprint provides strong evidence that the increased CO2 in the atmosphere originates from fossil fuels. Volcanoes contribute only about 1% of the CO2 emissions produced by human activities each year. Natural CO2 emissions are normally balanced by exchanges between the atmosphere, plants, and animals but the elimination of carbon sinks (such as forests and wetlands) has disturbed that balance. Other greenhouse gases, such as methane from farm animals and nitrous oxide from fertilizers, are also on the rise, further contributing to global warming.

Climate Model Simulations

Climate models are sophisticated computer simulations that use mathematical equations to represent the physical processes in the atmosphere, oceans, and land. These models are crucial tools for understanding past climate changes and projecting future climate scenarios. Climate simulations that only account for natural causes of climate change (solar variations, volcanic activity) predict either no change or a cooling trend in the 20th century, which is not what has occurred. Only when human factors, such as greenhouse gas emissions, are included do these models accurately reproduce the observed warming trend.

Observed Temperature Changes

Although some regions still experience cold temperatures, the frequency of record high temperatures has doubled since 2000, compared to record lows. This shift indicates a clear warming trend, with extreme heat events becoming more common. Each of the past three decades has been warmer than any other decade since record-keeping began in 1850. Since 1900, global temperatures have risen by nearly a degree Celsius (1.8 degrees Fahrenheit), with most of the increase occurring since the 1970s. Data from tree rings and ice cores suggest that the past 30 years have been the warmest in the last eight centuries, providing a long-term context for current warming trends.

Uneven Warming and Ocean Effects

Warming does not occur uniformly across the globe. Certain regions, such as the Arctic, are warming at a much faster rate than others. Oceans, covering over 70% of the Earth's surface, absorb more than 90% of the additional heat trapped by greenhouse gases, leading to significant changes in ocean conditions. This absorption moderates surface air temperatures but results in thermal expansion and changes in ocean currents and ecosystems.

Rising Sea Levels and Ocean Acidification

Global sea levels are rising by approximately a tenth of an inch per year and have increased by 8 inches since 1901. This rise is due to the thermal expansion of water as it warms and the melting of ice sheets and glaciers in Greenland and Antarctica. The melting of land ice adds water to the ocean, further contributing to sea-level rise. Rising sea levels threaten coastal communities and ecosystems.

The oceans, the Earth's largest carbon sink, are absorbing more CO2 from the atmosphere, causing the water to become more acidic. This process, known as ocean acidification, reduces the pH of the water. While the oceans will not become actual acid, the increased acidity endangers marine life, particularly animals with calcium-based shells (such as corals and shellfish), which are vital to marine ecosystems. Current projections indicate that the oceans could reach a pH of 7.8 within the next 100 years, potentially causing the extinction of about one-third of marine species and disrupting the entire marine food web.

Arctic Sea Ice Decline

Arctic summer sea ice has decreased by 40% since 1978, possibly reaching its lowest levels in 1,400 years. Satellite observations and historical data confirm this rapid decline. Sea ice reflects solar energy back into the atmosphere, but as it melts, the dark ocean absorbs more heat, accelerating the warming cycle. This feedback loop amplifies warming in the Arctic region, affecting weather patterns and ecosystems worldwide.

Future Projections

If current CO2 emission trends continue, the Earth is projected to warm by 2.5-5 degrees Celsius (4.5-9 degrees Fahrenheit), and sea levels could rise by up to a meter by the end of this century. These projections are based on climate models that incorporate various emission scenarios and feedback mechanisms. The actual extent of warming and sea-level rise will depend on future emissions and policy decisions.

Significance of Climate Change

These changes represent a significant threat with the potential of broad consequences, including more frequent and intense heatwaves, changes in precipitation patterns, increased risk of wildfires, and disruptions to agriculture and water resources. The last time the Earth was a few degrees colder, a mile-thick ice sheet covered much of North America. A similar magnitude of warming will lead to substantial global challenges, including displacement of populations, economic instability, and threats to global security.