Are humans responsible for climate change?

I'll gather the current science on humans and climate change.

Yes - according to the overwhelming scientific evidence and consensus, humans are responsible for most of the observed climate change, particularly global warming since the mid-20th century.

Scientific consensus

•There is unequivocal evidence that Earth is warming at an unprecedented rate. Human activity is the principal cause.

•The scientific consensus is that the Earth's climate system is unequivocally warming, and that it is extremely likely (meaning 95% probability or higher) that this warming is predominantly caused by humans.

•Multiple independent analyses find agreement in the high-90s percentiles, with studies reporting consensus figures from ~97% to greater than 99%.

•Specifically, 97% to 99.9% of climate scientists agree that climate change is happening and that human activity is the primary cause.

•Yes, the vast majority of actively publishing climate scientists – 97 percent – agree that humans are causing global warming and climate change.

What the major assessments say

The UN Intergovernmental Panel on Climate Change (IPCC), composed of scientific experts from countries all over the world, concluded that:

> Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1°C above 1850–1900 in 2011–2020.

> Observed increases in well-mixed GHG concentrations since around 1750 are unequivocally caused by GHG emissions from human activities.

> it is unequivocal that the increase of CO2, methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the principal driver of many changes observed across the atmosphere, ocean, cryosphere and biosphere.

> "It is unequivocal that human influence has warmed the atmosphere, ocean and land," wrote the authors of the IPCC's sixth global science assessment.

The IPCC also found that observed warming is human-caused, with warming from greenhouse gases (GHG), dominated by CO2 and methane, partly masked by aerosol cooling.

How humans are causing it

•Scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the "greenhouse effect" — warming that results when the atmosphere traps heat radiating from Earth toward space.

•The greenhouse effect is essential to life on Earth, but human-made emissions in the atmosphere are trapping and slowing heat loss to space.

•Over the last century, burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2).

•The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels by nearly 50% since 1750. This increase is due to human activities, because scientists can see a distinctive isotopic fingerprint.

•Five key greenhouse gases are carbon dioxide, nitrous oxide, methane, chlorofluorocarbons, and water vapor, with CO2, methane and nitrous oxide now at levels unprecedented in at least 800,000 years.

Is it just natural variation or the Sun?

Natural factors do affect climate, but they cannot explain current warming:

•While the Sun has played a role in past climate changes, the evidence shows the current warming cannot be explained by the Sun.

•Since 1750, the average amount of energy from the Sun either remained constant or decreased slightly.

•Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.

Quantitatively, the IPCC assessed:

> The likely range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C. ... natural (solar and volcanic) drivers changed global surface temperature by ±0.1°C and internal variability changed it by ±0.2°C.

In other words, essentially all of the warming observed between 1850–1900 and 2010–2019 is attributed to humans, while natural drivers contributed near zero in the same period.

Therefore, while climate has changed naturally in the distant past, the rapid warming observed since about 1950 is unequivocally dominated by human activities, principally greenhouse gas emissions from burning fossil fuels, deforestation, agriculture, and industrial processes.

Citations (3)

1nasa.govGovernment
Climate Change - NASA Science
Backs: “There is unequivocal evidence that Earth is warming at an unprecedented rate.”
2ipcc.chGovernment
Summary for Policymakers
Backs: “The UN Intergovernmental Panel on Climate Change (IPCC), composed of scientific experts from countries all over the world, concluded that: Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1°C above 1850–1900 in…”
3nasa.govGovernment
Causes - NASA Science
Backs: “Observed increases in well-mixed GHG concentrations since around 1750 are unequivocally caused by GHG emissions from human activities. it is unequivocal that the increase of CO2, methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the…”