10.2 Atmospheric concentration trends of greenhouse gases

Greenhouse Gas Concentration Trends

Since the Industrial Revolution, the concentrations of the three major greenhouse gases have climbed well beyond anything in the natural record. Understanding these trends, what's driving them, and how we know about past levels is central to climate science.

Concentration trends since the Industrial Revolution

Before industrialization, $CO_2$ sat at roughly 280 ppm. Today it's surpassed 420 ppm, an increase of about 50%. That rise is not gradual and steady; the rate has accelerated in recent decades, with $CO_2$ currently climbing around 2 ppm per year.

The other major greenhouse gases show similar patterns:

• Methane ($CH_4$) has more than doubled, from pre-industrial levels of roughly 700 ppb to over 1,900 ppb today.
• Nitrous oxide ($N_2O$) has risen about 20%, from roughly 270 ppb to over 335 ppb.

All three gases are increasing faster now than they were a few decades ago.

What's driving the increase

The post-industrial rise in greenhouse gases traces back to specific human activities:

• Fossil fuel combustion is the single largest source of added $CO_2$. Burning coal, oil, and natural gas for electricity, heat, and transportation releases carbon that was locked underground for millions of years.
• Deforestation and land-use change reduce the planet's capacity to pull $CO_2$ out of the air. Clearing the Amazon rainforest or draining Indonesian peatlands removes carbon sinks and often releases stored carbon at the same time.
• Agriculture is a major source of $CH_4$ and $N_2O$. Ruminant livestock like cattle produce $CH_4$ during digestion, and flooded rice paddies generate $CH_4$ through anaerobic decomposition. Heavy fertilizer use on cropland drives $N_2O$ emissions.
• Industrial processes such as cement production and chemical manufacturing release $CO_2$ and other greenhouse gases as byproducts.
• Waste management contributes as well. Landfills emit $CH_4$ as organic waste decomposes, and wastewater treatment releases both $CH_4$ and $N_2O$.

How we know about past concentrations

Scientists reconstruct ancient atmospheric composition using ice cores, cylinders of ice drilled from glaciers in Antarctica and Greenland. As snow compacts into ice over thousands of years, tiny air bubbles get sealed inside, preserving samples of the atmosphere from the time the snow fell.

Two key ice core records, the Vostok core and the EPICA Dome C core, extend back roughly 800,000 years. They show that:

• $CO_2$ naturally fluctuated between about 180 ppm during glacial (ice age) periods and about 280 ppm during warmer interglacial periods. Today's 420+ ppm is far outside that natural range.
• $CH_4$ ranged from roughly 350 to 800 ppb. Today's 1,900+ ppb is more than double the highest natural peak.
• $N_2O$ ranged from about 200 to 300 ppb, making today's 335+ ppb clearly above the historical ceiling.

Current concentrations of all three gases are unprecedented in at least 800,000 years of Earth's history.

Emissions vs. atmospheric accumulation

Not all the greenhouse gases humans emit stay in the atmosphere. About half of the $CO_2$ we release gets absorbed by carbon sinks, primarily the oceans and terrestrial ecosystems like forests and soils. The other half accumulates in the atmosphere, which is why concentrations keep rising year after year.

The relationship between emissions and concentrations isn't perfectly linear, though, because of complex interactions within the Earth system. Feedback loops can amplify the effect of our emissions:

• Thawing permafrost releases stored $CO_2$ and $CH_4$, adding to concentrations beyond what human sources alone would produce.
• Reduced ice and snow cover lowers Earth's albedo (reflectivity), causing more warming, which can further reduce the effectiveness of carbon sinks.

These feedbacks mean that the atmosphere's response to our emissions can be larger than a simple one-to-one calculation would predict.