How Do We Know Climate Change Is Real?

Image: iStock

To a certain extent, climate change is a natural occurrence - something that has happened throughout the history of our planet. Looking at the last 650,000 years alone, there have been seven cycles of glacial advance and retreat, and with the end of the last ice age about 7,000 years ago the modern climate era begun.

Historically, most climate changes are caused by tiny variations in Earth's orbit, which in turn change the amount of solar energy the planet receives. So how do we know what we are currently seeing in regards to current climate change is even remotely likely to be human-induced? Could it not just be a part of the natural cycle?

Image: NASA

As NASA points out, the current warming trend is "of particular significance, because most of it is very likely human-induced and proceeding at a rate that is unprecedented in the past 1,300 years".

We know this because through Earth-orbiting satellites and other technological advances enabling scientists to "see the big picture", collecting many different types of information about our planet and its climate on a global scale.

"This body of data," NASA says, "collected over many years, reveals the signals of a changing climate".

"The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause the Earth to warm in response."

Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth's climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly ten times faster than the average rate of ice-age-recovery warming.

NASA point to the following as compelling evidence of "rapid" climate change - a comprehensive list of sources are included for reference:

Sea level rise

Image: NASA

Global sea level rose about 17 centimeters in the last century. The rate in the last decade, however, is nearly double that of the last century.

NASA have a chart available here, which is updated monthly.

Global temperature rise

All three major global surface temperature reconstructions show that Earth has warmed since 1880. Most of the warming occurred in the past 35 years, with 15 of the 16 warmest years on record occurring since 2001. The year 2015 was the first time the global average temperatures were 1 degree Celsius or more above the 1880-1899 average.

Even though the 2000s witnessed a solar output decline resulting in an unusually deep solar minimum in 2007-2009, surface temperatures continue to increase.

In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change, a group of 1,300 independent scientific experts from countries all over the world under the auspices of the United Nations, concluded there is a more than 95 per cent probability that human activities over the past 50 years have warmed our planet.

The industrial activities that our modern civilisation depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million in the last 150 years. The panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in Earth's temperatures over the past 50 years.

Warming oceans The oceans have absorbed much of this increased heat, with the top 700 meters of ocean showing warming since 1969.

Shrinking ice sheets

The Greenland and Antarctic ice sheets have decreased in mass. Data from NASA's Gravity Recovery and Climate Experiment show Greenland lost 150 to 250 cubic kilometers (36 to 60 cubic miles) of ice per year between 2002 and 2006, while Antarctica lost about 152 cubic kilometers (36 cubic miles) of ice between 2002 and 2005.

This interactive ice viewer shows the decline.

Declining Arctic sea ice

Both the extent and thickness of Arctic sea ice has declined rapidly over the last several decades.

NASA's Operation IceBridge images Earth's polar ice in unprecedented detail to better understand processes that connect the polar regions with the global climate system.

Here's a whole bunch of peer-reviewed studies from the IceBridge program.

Glacial retreat

Glaciers are retreating almost everywhere around the world — including in the Alps, Himalayas, Andes, Rockies, Alaska and Africa.

Extreme events

The number of record high temperature events in the United States has been increasing, while the number of record low temperature events has been decreasing, since 1950. The U.S. has also witnessed increasing numbers of intense rainfall events.

The official website for NASA's fleet of Earth science missions that study rainfall and other types precipitation around the globe releases regular information in the form of images, data and videos.

Ocean acidification

Since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30 per cent. This increase is the result of humans emitting more carbon dioxide into the atmosphere and hence more being absorbed into the oceans. The amount of carbon dioxide absorbed by the upper layer of the oceans is increasing by about 2 billion tons per year.

NOAA shows how the CO2 absorbed by the ocean is changing the chemistry of the seawater, and the effect this is having.

Decreased snow cover

Satellite observations reveal that the amount of spring snow cover in the Northern Hemisphere has decreased over the past five decades and that the snow is melting earlier.

Here's a global map for reference.


IPCC Fourth Assessment Report, Summary for Policymakers, p. 5

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In the 1860s, physicist John Tyndall recognized the Earth's natural greenhouse effect and suggested that slight changes in the atmospheric composition could bring about climatic variations. In 1896, a seminal paper by Swedish scientist Svante Arrhenius first predicted that changes in the levels of carbon dioxide in the atmosphere could substantially alter the surface temperature through the greenhouse effect.

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L. Polyak,, “History of Sea Ice in the Arctic,” in Past Climate Variability and Change in the Arctic and at High Latitudes, U.S. Geological Survey, Climate Change Science Program Synthesis and Assessment Product 1.2, January 2009, chapter 7

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National Snow and Ice Data Center

World Glacier Monitoring Service

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National Snow and Ice Data Center

C. Derksen and R. Brown, "Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections," GRL, 39:L19504

Rutgers University Global Snow Lab, Data History Accessed August 29, 2011.]

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