9.5 Global Climate Change

Climate is the long-term average of weather in a place. Earth's climate has shifted many times due to natural causes: slow cycles in Earth's orbit and tilt that set the timing of ice ages, changes in the sun's output, large volcanic eruptions and the slow movement of continents.

What makes today's change unusual is its cause and its speed. Global average temperature has risen about 1.2 °C (roughly 2 °F) since the late 1800s, most of it since the 1970s, and recent years have been the warmest on record. Scientific assessments conclude this warming is driven mainly by human greenhouse gas emissions.

Ice sheets in Antarctica and Greenland are built from snow that piled up year after year and was compressed into ice. As it compressed, it sealed in tiny bubbles of air. Scientists drill out long cylinders called ice cores. The air bubbles record the CO₂ and methane in the atmosphere when the ice formed, and the ratio of heavy to light isotopes (forms of the same element with different masses) of hydrogen and oxygen in the ice records the temperature at that time.

Antarctic ice cores go back about 800,000 years. They show that CO₂ and temperature rose and fell together through repeated ice ages and warm periods. During ice ages CO₂ was around 180 ppm, and during warm periods around 280 to 300 ppm. Today's level, over 420 ppm, is far higher than any point in that record. Other records of past climate include tree rings, layers of lake and ocean sediment, pollen and corals.

• Melting ice: mountain glaciers are retreating worldwide, Arctic sea ice is shrinking, and the Greenland and Antarctic ice sheets are losing mass.
• Thawing permafrost: permafrost is ground that stays frozen year-round in polar and high-mountain regions. It holds roughly twice as much carbon as the atmosphere does now, in frozen dead plant matter. When it thaws, microbes decompose that matter and release CO₂ and methane. Thawing also damages buildings, roads and pipelines built on it.
• Rising sea levels and coastal flooding that can displace people (topic 9.4).
• More extreme weather: more frequent and intense heat waves, heavier downpours, and more severe droughts in some regions.
• Changes in ocean currents and winds: freshwater from melting Greenland ice could slow the ocean circulation that carries warm water north in the Atlantic, which would shift regional climates.
• Ecosystem shifts: species moving toward the poles or uphill, and earlier spring events like flowering and migration that can fall out of sync with food supplies.

A positive feedback loop is one where a change triggers effects that make the original change bigger. The Arctic has warmed roughly three to four times as fast as the global average in recent decades because of feedbacks like these:

Ice-albedo feedback: albedo is how much light a surface reflects. Snow and ice are bright and reflect most sunlight. As they melt, they expose darker ocean or land, which absorbs more sunlight, which causes more warming and more melting.

Permafrost feedback: warming thaws permafrost, which releases CO₂ and methane, which causes more warming and more thawing.

A negative feedback loop works the other way and dampens a change. For example, more CO₂ can speed up plant growth in some places, pulling a bit more CO₂ out of the air. The positive feedbacks currently outweigh the negative ones.