Are humans responsible for climate change?

Are humans driving climate change? I'll gather the latest scientific evidence to answer.

Yes - according to the overwhelming scientific consensus and evidence, human activity is the principal cause of the climate change observed since the mid-20th century.

While Earth's climate has always changed due to natural factors, the current warming is anthropogenic - meaning human-caused.

1. There is an overwhelming scientific consensus

> Scientific consensus identifying humans as primarily responsible for climate change is not new and was already forming in the 1980s. Today, 97 percent to 99.9 percent of climate scientists agree that climate change is happening and that human activity is the primary cause

Other syntheses report the same result:

The U.S. space agency NASA summarizes:

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

2. What the IPCC says

The Intergovernmental Panel on Climate Change (IPCC), which synthesizes research from thousands of scientists worldwide, has strengthened its conclusion over decades:

NASA's summary of that report:

> in its Sixth Assessment Report, the Intergovernmental Panel on Climate Change, composed of scientific experts from countries all over the world, concluded that 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

3. The evidence linking humans to warming

a) The greenhouse effect has been amplified:

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

NASA explains:

> On Earth, human activities are changing the natural greenhouse. Over the last century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO2)

Detailed on NASA's Causes page:

> 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 in the atmosphere

The IPCC states human activities have caused about 1.1°C of warming above pre-industrial (1850-1900) levels by 2011-2020, and that human influence is clear.

b) Natural causes cannot explain current warming:

It is important to distinguish:

> It results from two sources: natural climate change, which consists of the natural climate cycles that have always occurred throughout Earth's history and anthropogenic climate change, which is caused by human activities

> Earth’s climate has always changed, but anthropogenic climate change is specifically about the recent, rapid change linked to industrialization, land-use change, and energy use

Current warming cannot be explained by the Sun:

> Since 1750, the average amount of energy from the Sun either remained constant or decreased slightly
> If a more active Sun caused the warming, scientists would expect warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere and a warming at the surface

> While Earth’s climate has always evolved due to natural forces such as volcanic activity, variations in solar radiation, and changes in Earth’s orbit, the climate change we are witnessing today is overwhelmingly driven by human activity

In short: Natural variability exists - ice ages, volcanic eruptions, solar cycles and other factors have driven climate change in the past - but multiple lines of evidence (rising greenhouse gas concentrations with a human isotopic signature, observed warming pattern in the atmosphere, climate models, and direct satellite measurements of trapped heat) show the rapid warming since the mid-1900s is dominated by human emissions of carbon dioxide, methane, nitrous oxide and other heat-trapping gases from burning fossil fuels, deforestation, agriculture and industry.