5.12: Future Geographies - The Evidence for Climate Change

Global Warming and Climate Change

It is clear from past research that Earth's climate has varied significantly over time, the causes of which are many and complex. Even over the span of human history, the Earth has undergone significant periods of warming and cooling. Present day global warming however is not, to most scientists, a consequence of natural climate variability. Most recognize that the present change to a warmer climate is a product of human impact on the Earth system.

Evidence for Global Warming

Though a few scientists remain skeptical, there is a a growing consensus that the present day warming is real and humans are driving it. Evidence for global warming can be found in every part of the Earth system. Besides well documented changes in air temperature, global warming is

• heating the world's oceans
• reducing sea ice extent, especially in the Arctic
• melting glaciers
• causing sea level to rise
• altering habitats and,
• affecting plant and animal distributions

The Natural Greenhouse Effect

Naturally occurring atmospheric gases—primarily water vapor, carbon dioxide (CO₂), and methane (CH₄)—are transparent to incoming shortwave solar radiation but highly absorbent of outgoing longwave infrared radiation. These gases absorb the escaping thermal energy and re-radiate it in all directions, including back down toward the surface.

Key Baseline: This natural retention of heat is vital for life. Without an atmosphere containing greenhouse gases, Earth’s calculated average surface temperature would be a frozen -18⁰ C (0⁰ F). The natural greenhouse effect raises this global average to a life-sustaining 15⁰ C  (59⁰ F).

The Anthropogenic Multiplier

Since the onset of the Industrial Revolution (circa 1750), the widespread combustion of fossil fuels has unnaturally accelerated the concentration of atmospheric carbon dioxide. By combining buried carbon with atmospheric oxygen, industrial activities have heavily amplified the atmosphere's thermal absorption capacity. This enhanced greenhouse effect restricts the escape of longwave radiation, systematically driving up mean global temperatures.

Global Warming vs. Climate Change

In scientific literature, environmental tracking requires distinct terminology to identify the root cause versus the cascading systemic consequences.

• Global Warming: Refers strictly to the observed, long-term increase in Earth’s average surface temperature since the pre-industrial era (1850–1900) due to human greenhouse gas emissions.
• Climate Change: A broader term encompassing global warming plus the full spectrum of secondary system responses. These include changing precipitation zones, migrating climate belts, shrinking glaciers, and shifts in ecosystem blooming seasons.

Paleoclimate archives—derived from ocean sediment cores and deep ice sheet layers—demonstrate that Earth's climate has historically fluctuated over millions of years. These natural adjustments were primarily driven by slow orbital cycles and slight variations in solar output.

Table \(\PageIndex{1}\): Comparing warming magnitude, duration, and speed

Climate Transition Period	Magnitude of Warming	Approximate Duration	Average Velocity of Change
Post-Glacial Recovery (Historical Ice Age Exit)	4^0C to 7^0C	approx 5,000 years	approx 0.01^0C per century
Modern Anthropogenic Era (Post-Industrial Shift)	approx 1^0C total since 1880	Ongoing	0.2^0C per decade

Oceanic Regulation of Climate

Covering roughly 70% of Earth's surface, the world's ocean serves as the primary regulator of the global climate system. This immense stabilizing influence is due to water's exceptionally high heat capacity, meaning the top few meters of the ocean store as much thermal energy as Earth's entire atmosphere.

Thermal Transport via Oceanic Currents

Because the planet is a sphere, equatorial regions receive significantly more direct solar radiation than polar latitudes. The ocean works to equalize this energy imbalance through a massive, interconnected circulation network known as the global ocean conveyor belt. Warm surface currents transport excess tropical heat northward toward the colder poles. As this water approaches the polar zones, it releases its heat into the atmosphere, cools, increases in salinity and density, and sinks. This dense, cold water then flows back toward the equator along the deep ocean floor, establishing a continuous global heat distribution loop.

Climate Dampening and Sea Level Drivers

Because the ocean acts as Earth's primary heat sink, it has successfully absorbed approximately 90% of the excess thermal energy trapped by anthropogenic global warming over recent decades. While this buffers the atmosphere against immediate, extreme air temperature spikes, it drives substantial oceanic change:

1. Thermal Expansion: As water molecules absorb excess thermal energy, they expand and increase in volume, causing ocean levels to rise.
2. Continental Ice Influx: Accelerated global warming melts land-bound glaciers and massive polar ice sheets (such as Greenland and Antarctica), discharging massive volumes of meltwater into the ocean basin.

Evidence from the oceans

Global warming has already had a significant impact on the hydrosphere, especially glaciers and oceans. Tide gauge measurements show a worldwide increase of sea level of 15-20 cm (6-8 inches). According to the Intergovernmental Panel on Climate Change (IPCC) suggest the rise is due to the expansion of ocean water due to rising temperatures (contributing about 3 - 7 cm) and the melting of mountain glaciers and small ice caps (contributing about 2 - 4 cm). Sea level is rising so high in locations like the Arctic coast of Canada that small villages are threatened.

A 2005 study provided "compelling" evidence that ocean warming over the past 40 years is linked to the industrial release of carbon dioxide. Scientists from Scripps Institution of Oceanography used a variety of scenarios to reproduce the observed rise of ocean temperatures over the last four decades. The rise of ocean temperatures could not be accurately explained by computer models based on the natural climate variability of solar radiation and volcanic emissions. "What absolutely nailed it was greenhouse warming," said Dr Barnett, lead scientist on the study. Their model reproduced the observed ocean temperatures with a statistical confidence of 95%.

Arctic sea ice has experienced a significant decrease in areal extent over the past few decades. Recent NASA research indicates that Arctic sea ice is shrinking at a rate of 9.6 percent per decade. September ice coverage between 2000-2005 were 20% below the period 1979 - 2000. Such ice thinning and retreat has impacts ocean salinity, heat balance and animal habitat. Polar bear populations are on the decline as ice thins making for precarious hunting conditions. Ice pack break up leaves polar bears stranded further from land. Polar bears are being forced to swim longer distances between ice floes and drowning during the journey.

Evidence from glaciers

Evidence for warming is occurring in all parts of the Earth system. Most of the world's glaciers are in retreat as a result of global warming. Evidence for the loss of ice is documented in the journals of the first explorers to Alaska. In the 1790's travelers to the area near Glacier Bay reported only a small embayment of coastline with a large glacier occupying the basin of Glacier Bay. By the 1890's the glacier was in retreat as 40 miles of coastline was now exposed. Today, visitors can still marvel at the great tidewater glaciers, but for how long? Glacier Bay now extends for 60 miles.

Figure \(\PageIndex{1a}\): Muir Glacier, 1899. (CC BY-SA 4.0; Michael Ritter via The Physical Environment).

Figure \(\PageIndex{1b}\): Muir Glacier, 2003. (CC BY-SA 4.0; Michael Ritter via The Physical Environment).

A first ever gravity survey of Antarctica recently revealed that it had lost a substantial amount of mass. Researchers found that the ice sheet covering Antarctica lost 152 (plus or minus 80) cubic kilometers of ice annually between April 2002 and August 2005. The estimated mass loss was enough to raise global sea level about 1.2 millimeters (0.05 inches), about 13 percent of the overall observed sea level rise for the survey period.

Evidence from plant and animal distributions

Evidence of global warming is appearing in the distribution of plant and animal species. Growing seasons in many parts of the globe have shifted as a result of warmer temperatures. Scientists have observed that plants have been slowly moving toward higher elevations in some mountain regions. This "escalator effect" as it has been called, has pushed some plant species up as much as ten feet (3 meters) in some mountain regions. Because some plant species migrate faster than others, established ecosystems could be severely altered.

Figure \(\PageIndex{2}\): Great Tit (Parus major). (CC BY-SA 4.0; Michael Ritter via The Physical Environment).

Animal species are also responding to habitat changes as a result of global warming. As spring arrives earlier in the year, migrating animals start moving earlier in search of food sources. A recent study of the great tit (Parus major) reported that the birds are laying eggs nearly two weeks earlier than they have over the past fifty years. Doing so enables them to hatch earlier and take advantage of caterpillars which are also appearing two weeks earlier due to warmer spring temperatures.

Geographic Patterns of Global Warming

The effects of global warming will not be the same in all places. The smallest changes in temperature are to occur in tropical regions, while the Arctic and Antarctic will experience considerable changes. The Arctic regions are seen as the "bell weather" of what global warming will bring. 

Figure \(\PageIndex{3}\): House damaged by melting permafrost. (CC BY-SA 4.0; Michael Ritter via The Physical Environment).

The Arctic is already experiencing the impact of global warming. The melting tundra presents an engineering nightmare to those living in this environment. Coastal erosion of permfrost is destabilizing slopes causing them to fail. Climate scientists fear that the melting permafrost will release millions of tons of stored carbon back into the atmosphere further fueling global warming.