Burning fossil fuels releases carbon dioxide and other heat-trapping gases into the atmosphere, driving a cascade of changes to the climate, oceans, air quality, and living ecosystems. The concentration of CO2 in the atmosphere has climbed to roughly 429 parts per million, up from about 280 ppm before the Industrial Revolution. That single shift, more CO2 overhead, is the root cause of most environmental and health consequences that follow.
What Combustion Actually Produces
Every fossil fuel, whether coal, oil, or natural gas, is built on chains of carbon atoms. When you burn any of them, the carbon combines with oxygen in the air to form carbon dioxide. That reaction also releases energy, which is the whole point of burning fuel in the first place. But CO2 is only one product. The extreme heat inside engines and power plants (several hundred degrees or more) forces nitrogen and oxygen molecules already in the atmosphere to react with each other, creating nitrogen oxides. Coal and some oil also contain sulfur impurities, which burn into sulfur dioxide. On top of those gases, incomplete combustion throws off tiny particles of soot and other fine particulate matter.
Not all fossil fuels produce the same amount of CO2. Per unit of energy, coal is the most carbon-intensive: anthracite coal releases about 229 pounds of CO2 per million BTU. Gasoline comes in around 157 pounds, and natural gas is the lowest at 117 pounds. That difference matters for electricity generation and transportation, but every fossil fuel adds carbon to the atmosphere that had been locked underground for millions of years.
How CO2 Traps Heat
Earth’s surface absorbs sunlight and radiates that energy back toward space as infrared radiation. CO2 molecules in the atmosphere intercept some of that outgoing infrared energy. When a CO2 molecule absorbs an infrared photon, it vibrates faster, and that extra kinetic energy heats the surrounding air. The molecule eventually re-emits the energy as another infrared photon, but in a random direction, so roughly half of that energy heads back down toward Earth’s surface rather than escaping to space. This is the greenhouse effect, and it is the physical mechanism behind global warming.
The process is not speculative. It has been understood in laboratory physics since the 1800s, and satellites measure the exact wavelengths of infrared radiation being absorbed by CO2 in real time. More CO2 means more absorption, more re-emission downward, and a warmer planet.
Rising Temperatures and Sea Levels
By the decade 2006 to 2015, human activity had already warmed the planet by about 0.87°C compared to the pre-industrial baseline of 1850 to 1900. Warming has accelerated since then. That may sound small, but global averages mask regional extremes: the Arctic warms two to three times faster than the global mean, and heat waves that were once rare now occur more frequently across every continent.
Warmer temperatures melt glaciers and ice sheets, and warmer water expands in volume. Both effects raise sea levels. Since the early 1990s, the ocean has risen at a long-term average rate of about 0.17 inches (0.44 centimeters) per year, and the overall rate of annual sea level rise has more than doubled compared to earlier decades. Individual years fluctuate (2025 saw a temporary dip due to La Niña conditions), but the trend line is consistently upward. Coastal flooding, shoreline erosion, and saltwater intrusion into freshwater supplies are already affecting communities worldwide.
Ocean Acidification
The ocean absorbs roughly a quarter of the CO2 humans emit, which sounds helpful for the atmosphere but creates a serious chemical problem underwater. When CO2 dissolves in seawater, it reacts with water molecules to form carbonic acid. That acid lowers the ocean’s pH. Before the 1700s, average ocean pH sat around 8.2. Today it is about 8.1. A single decimal point may seem trivial, but pH is a logarithmic scale: that shift represents a 25% increase in acidity, a pace faster than any known change in Earth’s geologic past.
Higher acidity makes it harder for shellfish, corals, and tiny plankton to build their calcium carbonate shells and skeletons. Coral reefs, which support roughly a quarter of all marine species, are especially vulnerable. As the base of ocean food webs weakens, the effects ripple upward through fisheries that billions of people depend on for protein.
Air Quality and Human Health
The same combustion that releases CO2 also fills the air with pollutants that directly harm human bodies. Fine particulate matter, particles smaller than 2.5 micrometers (known as PM2.5), is small enough to pass through the lungs and enter the bloodstream. Long-term exposure damages respiratory airways, triggers chronic inflammation, and impairs lung function. Nitrogen dioxide, another combustion byproduct, irritates airways and worsens asthma.
The toll is enormous. A 2023 study published in the BMJ estimated that about 5.1 million excess deaths per year globally are attributable to outdoor air pollution from fossil fuel use. That figure accounts for heart disease, stroke, lung cancer, and respiratory illness linked to breathing polluted air. These deaths are concentrated in regions with heavy coal use and dense traffic, but air pollution crosses borders: particulate matter can travel hundreds of miles from its source.
Effects on Wildlife and Ecosystems
Climate change driven by fossil fuel emissions is reshaping habitats faster than many species can adapt. The proportion of observed global extinctions attributed to climate change has been increasing by about 4% per decade since 1970. A comprehensive analysis published in Science projected that, averaged across all emissions scenarios, global climate change threatens roughly 7.6% of species with extinction. Under the highest-emission scenario, that figure climbs to approximately one-third of all species.
Species with small, localized ranges are at the greatest risk. Studies focusing on endemic species, those found only in one area, estimated extinction risks around 10 to 12%. When a mountaintop species has nowhere cooler to migrate, or when a coral reef organism cannot tolerate even slight warming, the math becomes unforgiving. Beyond outright extinction, shifting temperature zones force animals and plants to move toward the poles or to higher elevations, disrupting pollination cycles, predator-prey relationships, and food availability in ways that cascade through entire ecosystems.
Why the Effects Compound Over Time
CO2 lingers in the atmosphere for hundreds of years. Every ton emitted today will still be trapping heat in the year 2300. That means the warming, acidification, and ecological disruption happening now reflect emissions from decades past, and the full consequences of today’s emissions have not yet arrived. Ice sheets respond on timescales of centuries. Ocean currents redistribute heat slowly. Species adapt over generations, not years.
This lag effect is one of the most important things to understand about fossil fuel combustion. The climate system does not reset when emissions slow down. It accumulates. The total amount of CO2 already in the atmosphere, around 429 ppm and climbing, commits the planet to continued warming even if emissions were to stop tomorrow. The speed and scale of future impacts depend on how much additional CO2 enters the atmosphere from this point forward.