HOW DO WE KNOW HUMANS ARE CAUSING CLIMATE CHANGE?

Yes, we know humans are responsible for the climate change we see today. But here’s the good news: Human-caused climate change can be human-solved climate change.

Put simply, here’s what we know for sure:

• We know that carbon dioxide (and other greenhouse gases) trap heat in the Earth’s atmosphere.
• And we know that humans are burning fossil fuels, releasing huge amounts of carbon pollution and trapping more and more heat in the atmosphere.
• There’s only one conclusion: humans are the primary reason we see our climate changing today.

Unprecedented warming

Earth's surface has undergone unprecedented warming over the last century, and especially in this century.

Every single year since 1977 has been warmer than the 20th century average, with 16 of the 17 warmest years on record occurring since 2001, and 2016 being the warmest year on recorded history. A study from 2016 found that without the emissions from burning coal and oil, there is very little likelihood that 13 out of the 15 warmest years on record would all have happened.

As any farmer can tell, the natural patterns of climate have been altered.

We know that warming—and cooling—has happened in the past, and long before humans were around. Many factors (called “climate drivers”) can influence Earth’s climate—such as changes in the sun’s intensity and volcanic eruptions, as well as heat-trapping gases in the atmosphere.

But, what’s causing today’s unprecedented warming — are humans part of the cause?

Scientists have devised different methods to answer this question. Meteorologists and oceanographers compare the climate patterns they observe with patterns developed using sophisticated models of Earth's atmosphere and ocean. By matching the observed and modeled patterns, scientists can positively identify the "human fingerprints" associated with the changes, and they can also attribute the proportion of those changes to human activities.

The fingerprints that humans have left on Earth's climate are turning up in a diverse range of records and can be seen in the ocean, in the atmosphere, and on the Earth’s surface.

Scientists agree that today’s warming is primarily caused by humans putting too much carbon in the atmosphere, like when we choose to extract and burn coal, oil, and gas, or cut down and burn forests.

Scientists have gathered evidence and have improved their methods for teasing apart natural and human factors. Today scientists have very high confidence about human-caused global average surface temperature increase – a key climate indicator. They have reported on their growing confidence through successive climate assessments of the Intergovernmental Panel on Climate Change (IPCC).

Direct evidence of human contribution to atmospheric CO2

Carbon dioxide concentrations.

Carbon dioxide (CO2) is the main heat-trapping gas largely responsible for most of the average warming over the past several decades.

The atmospheric concentration of CO2 has increased dramatically, from a pre-industrial era (AD 1000 – 1750) concentration of approximately 280 parts per million (ppm) to today's 400 ppm.

Scientists warned for years about this dangerous threshold, but with the accelerated pace of emissions the question changed from whether we would reach CO2 concentrations above 400ppm to when.

The Arctic reached 400 ppm in 2012. In 2013 the Mauna Loa Observatory in Hawaii recorded more than 400ppm. In March 2015 global averages reached this threshold, and in September 2016 the world reached a point of no-return: CO2 concentration levels are unlikely to dip below 400 ppm again.

Direct Evidence of Fossil Fuel Derived CO2 in the Atmosphere. While the concentration of carbon has increased, the carbon originating from natural sources has decreased. 

We know human activities are driving the increase in CO2 concentrations because atmospheric CO2 contains information about its source. Scientists can tease apart how much CO2 comes from natural sources, and how much comes from combusted fossil fuel sources.

Compared to other carbon sources, carbon from fossil fuels has a distinctly different “signature,” essentially the relative amount of heavier or lighter atoms of carbon (technically δ13C). The more negative the δ13C, the higher the proportion of carbon from fossil fuels.

Over the years, δ13C has decreased while the overall amount of CO2 has increased. This information tells scientists that fossil fuel emissions are the largest contributor of CO2 concentrations since the pre-industrial era.

Natural and human factors that influence the climate (known as “climate drivers”)

Many natural and human factors (also called drivers) influence the climate.

Emissions from cars and power plants and an increase in the amount of radiation the sun emits are examples of "forcings" that drive temperature rise, the first one by trapping heat, and the second one by increasing energy, which translated into heat. Volcanic events and some types of human-made pollution, both of which inject sunlight-reflecting aerosols (i.e., tiny particles) into the atmosphere, lower temperature and are examples of forcings that drive decreases in temperature.

A recent study found that “almost two-thirds of the impacts related to atmospheric and ocean temperature can be confidently attributed to anthropogenic forcing” (meaning human caused drivers).

Human activity drives climate change.

Natural climate drivers include the energy from the sun; aerosols from periodic volcanic eruptions, dust, and salt spray; natural carbon cycle processes like termite mounds in Africa that emit methane, or tiny organisms in the ocean surface that take up carbon dioxide; and variation in snow and ice cover that change how much the Earth’s surface reflects the sun’s energy back into space (referred to as albedo).

History of Climate Drivers: Heat-trapping emissions far outweigh the effects of other drivers acting on Earth’s climate. Volcanic eruptions account for the cooling spikes seen in the graph in 1883 and 1991.

Among natural drivers, a large volcanic eruption can have a sharp cooling influence as it spews tiny particles high into the stratosphere (the layer of the atmosphere above the troposphere where weather typically occurs).

The massive explosions from Krakatoa(Indonesia) in 1883 and Mount Pinatubo(Philippines) in 1991, for example, can be seen as the two largest downward spikes in the volcanic data depicted in the figure to the right. These particles prevented the full energy of the sun from reaching the surface of Earth and created a cooling trend for several years.

Human climate drivers include heat-trapping emissions from burning coal, gas and oil in power plants and cars; cutting down and burning forests; tiny pollution particles (aerosols); black carbon pollution more commonly referred to as soot; and changes in land use that also affects Earth’s albedo.

Fossil fuel burning by humans emits tiny particles in addition to releasing CO2 in the atmosphere. Some particles reflect sunlight back to space (aerosols), similar to the volcanic particles, having a cooling effect.

Other particles such as soot (black carbon) absorb the sunlight and drive temperature rise, leading to local warming of the atmosphere level where the soot particles circulate. Both types of human-created particles lead to a decrease in the amount of the sun’s energy reaching the surface of the Earth.

Very likely, there would have been even more warming in the past 60 years if it were not for these human-made and natural tiny particles.