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How Do We Know That Global Warming Is Being Caused by Humans?

I have been studying the science of global warming with a passion since the late 1970s and earned a degree in Environmental Science in 1992.

What is causing global warming and what indicates humans are the primary cause of the warming we have been witnessing? These are important questions since understanding the causes can help us come up with viable solutions.

Is the warming we’ve experienced in recent decades simply being caused by natural variations in the Earth’s climate that have occurred since the Earth developed a climate billions of years ago or have mankind’s activities on Earth played a significant role in the global warming observed since the later part of the 20th century?

The answer is not opaque, it’s actually quite clear. Scientists have studied a number of climate indicators that point to human activities being the cause of recent global warming. Understanding these indicators is the key to understanding why the vast majority of scientists have concluded that our activities are the primary cause of the recently observed global warming and the changes in climate that the warming has been linked to. Let’s take a look at the indicators based scientific data.

The Greenhouse Effect

A depiction of the greenhouse effect and how it affects Earth

A depiction of the greenhouse effect and how it affects Earth

An Increase of a Carbon Dioxide’s Carbon-12 Isotope Is a Strong Clue

Carbon exists in three isotopes (different weights based on the number of neutrons in a carbon atom). 99% of carbon on Earth is carbon-12, which has 6 neutrons. Carbon-13 with 7 neutrons accounts for about 1%. Carbon-14, which has 8 neutrons, accounts for an infinitesimal amount of approximately 1 in 1 trillion carbon atoms.

The amount of carbon-12 versus carbon-13 (the carbon ratio) currently present in the Earth’s atmosphere within the compound carbon dioxide (CO2) is one of the strongest indications that mankind’s burning of fossil fuels is causing the global warming that we are experiencing. Fossil fuels primarily come from plants that decomposed while buried underground under immense pressure for millions of years. Plants prefer lighter carbon-12 for photosynthesis and have higher concentrations of carbon-12 than the Earth’s atmosphere traditionally has. Since fossil fuels are primarily derived from decomposed plants, they also have higher concentrations of carbon-12 than the Earth’s atmosphere normally has.

As fossil fuels are burned, they increase the ratio of carbon-12 versus carbon-13 in the atmosphere. This increase of carbon-12 in the carbon ratio has been documented in recent decades via direct measurements by scientists that research the composition of the atmosphere. Past carbon ratio data is derived from tree rings and ice cores.

The data indicate the carbon-12 versus carbon-13 ratio in the atmosphere has increased by approximately 0.15% since 1850. While this sounds like a rather small and insignificant increase, it is approximately five times greater than the documented carbon-12 versus carbon-13 ratio increase of 0.03% found in ice cores that cover a typical transition from an ice age (glacial) to a warm spell (interglacial) that takes thousands of years to occur. In other words, the documented increase of carbon-12 in the atmosphere since just 1850 far exceeds the natural rate of increase that has been documented when the Earth’s atmosphere warmed naturally in the past. This rapid increase in carbon-12 clearly points to a human cause rather than a natural cause for the observed increase in carbon in the atmosphere since 1850 and the associated atmospheric warming.

Earth's Temperature Record Going Back 12,000 Years

Scientists have collected data of the Earth's global average temperature going back 12,000 years

Scientists have collected data of the Earth's global average temperature going back 12,000 years

The Ice Age Cycle and What the Geologic Record Tells Us About Recent Global Warming

Ice ages (or what scientists call “glaciations”) have occurred approximately every 100,000 years over the past several million years. Inter-glacial warm periods, which occur between glaciations, typically last between 10,000 and 12,000 years. The current interglacial warm period that human civilization has developed in started approximately 11,300 years ago.

The temperature trends in the preceding graph indicate that global temperatures peaked about 7,000 years ago, then made a sustained move lower. The geologic record indicates we were heading into a new ice age right on schedule. This slide to a cooler regime was interrupted in recent decades, as global temperatures quickly moved well above the high mark set 7,000 years ago. This sudden spike in global temperatures which interrupted a sustained natural downtrend is another indication that the sudden warming we have been experiencing is not just natural variation.

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Earth Is Warming Faster Than It Naturally Did in the Past

The geologic record indicates that the Earth is currently warming far faster than it has in the past. So fast that a natural explanation makes no sense. For the global average temperature to rise by one degree Celsius in only a matter of approximately 100 years is quite unusual in comparison to natural warming observed in the geologic record. An indication that the recent warming is not likely due to natural variability.

A Depiction of Climate Drivers

The various climate drivers that have affected global temperatures in recent decades are depicted above

The various climate drivers that have affected global temperatures in recent decades are depicted above

Various Climate Drivers Don’t Account for Recent Warming, Except Mankind’s Release of Greenhouse Gasses

A climate driver is something that causes the climate to warm or cool over time. Climate drivers include the Sun’s energy output, land-use changes, aerosols (tiny particles) from volcanic eruptions, aerosols from industrial activities, and heat-trapping greenhouse gases. An examination of these climate drivers over the past 100 years indicates that only mankind’s release of heat-trapping greenhouse gases can account for the warming the Earth has experienced in recent years. Other climate drivers have either had a negative or minimally positive impact on global temperatures.

The Five Layers of the Atmosphere

The Tropopause is between the Troposphere and Stratosphere

The Tropopause is between the Troposphere and Stratosphere

The Troposphere Atmospheric Transition Zone Has Moved Further Out

The recent movement outward of the Troposphere Atmospheric Transition Zone (Tropopause), which is the boundary between the Troposphere and the Stratosphere, is yet another indicator of human-caused global warming. The Tropopause is usually at a consistent distance from the Earth’s surface because its distance is a function of the Sun’s radiation entering the Earth’s atmosphere. In recent decades, the Tropopause has migrated further away from the Earth’s surface due to global warming gases accumulating in the Troposphere that have changed the balance of the Sun’s radiation in the Troposphere, as greenhouse gases released by mankind build up and trap the Sun’s radiation.

How We Should Move Forward with Addressing Global Warming

Knowing that it has been conclusively established using scientific methods that mankind’s activities are the primary driver of global warming we’ve been experiencing, it is imperative that we quickly take actions to avoid allowing runaway global warming to take over and cause a massive spike in global temperatures that could seriously imperial mankind. Runaway global warming would occur once the Earth’s atmosphere warms to a point at which natural feedback loops such as melting frozen tundra take over and start releasing massive amounts of global warming gases carbon dioxide and methane into the atmosphere. Once released, these gases would cause additional warming, leading to additional melting and releases of gases, and on and on until the Earth is substantially warmer than it is today.

To address global warming, it is imperative that we take measures to ween human civilization off of fossil fuels and transition our societies to run off of non-carbon dioxide emitting renewable energy, and to take these measures as soon as possible. In light of the fact that we either might not have the will to change over to renewable energy or it might already be too late to do so, as future runaway global warming may already be inevitable, we should also start planning on implementing geoengineering solutions to stop and reverse global warming.

Further Reading About Geoengineering Solutions To Address Climate Change and Reverse Global Warming

Cause of Global Warming

What’s REALLY Warming the Earth?

What's causing global warming?

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 John Coviello

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