The current warming trend is moving at an unprecedented speed
In short: Earth has cycled through ice ages and warm periods for hundreds of thousands of years. However, the pace of our current warming is fundamentally different. While the planet has been hotter in the past, the speed at which we are heating up today is unlike anything seen in the last 10,000 years.
To understand why the current warming is unique, scientists look at the rate of change. Over the last 800,000 years, Earth experienced eight cycles of ice ages and warmer periods. Most of these shifts were driven by small variations in Earth's orbit. However, the warming since the mid-20th century is proceeding at a rate roughly 10 times faster than the average warming following an ice age. For context, the planet warmed by 5-8C during the Paleo-Eocene thermal maximum (PETM) about 56 million years ago, but that process took thousands of years. Today, we have seen a 0.85C increase since 1980, with projections suggesting a further 1.5C rise in just 40 years.
The mechanism behind this rapid heating is the greenhouse effect. Gases like carbon dioxide have at least three atoms in their molecules, which are loosely held and allow them to absorb vibrational energy from long-wave radiation (heat) bouncing off the Earth's surface. Unlike non-greenhouse gases, these molecules re-emit that radiation back toward the planet. While volcanoes also emit CO2, they produce only 1% as much as human activity. Furthermore, scientists can distinguish between the two by looking at carbon isotopes: fossil fuels contain higher ratios of C12, which is now being detected in higher concentrations in our atmosphere.
The evidence for human-driven warming is corroborated by multiple data streams. Data from satellites indicates that the lower atmosphere heats up whereas the upper layer does not, despite the sun's irradiance fluctuating by merely 0.15% during 11 years—a margin insufficient to account for modern patterns. When climate models use only natural causes, they predict no significant temperature change; it is only when human-driven emissions are added that the simulations match the observed data perfectly. This transition from theory to established fact was confirmed by the IPCC as the influence of human activity became clear in the 1970s.
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Sources
- Summary by Intellect · source: Is Climate Change Man Made? The Surprising Data!
- Also: Evidence - NASA Science
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