Are humans responsible for climate change?

I'll gather the latest scientific evidence to answer whether humans are responsible for climate change.

Yes — according to the overwhelming weight of scientific evidence, humans are responsible for modern climate change, as the principal or dominant cause.

1. What the major scientific bodies conclude

> “It is unequivocal that human influence has warmed the atmosphere, ocean and land”

> “It is unequivocal that human influence has warmed the atmosphere, ocean and land. ... The scale of recent changes ... are unprecedented over many centuries to many thousands of years.”

In its most recent Sixth Assessment cycle, the IPCC said with high confidence that human activities have “unequivocally caused global warming” — the strongest wording to date from any IPCC assessment.

The final synthesis report details the “unequivocal” role of humans in the 1.1C of temperature rise seen since the start of the industrial era.

Other assessments use the same language:

> “Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming”

> As NASA says, there is “unequivocal evidence that Earth is warming at an unprecedented rate” and that human activity “is the principal cause.”

And the IPCC summarized:

> “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.”

> “Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact.”

An earlier IPCC report put it at at least 95% certainty that human activities are the principal cause, while later reports moved to unequivocal and 95-100% (extremely likely) language.

2. How strong is the scientific consensus?

Multiple independent analyses of peer-reviewed literature and surveys of experts converge on the same finding:

> 97 percent to 99.9 percent of climate scientists agree that climate change is happening and that human activity is the primary cause

> Multiple independent analyses ... put agreement that humans are causing recent warming at roughly 97% or higher, with some large-scale literature reviews reporting consensus estimates above 99%

This consensus was already forming in the 1980s, and today leading scientific organizations worldwide have endorsed it. The U.S. National Academy of Sciences states scientists have known from multiple lines of evidence that humans are changing Earth's climate, primarily through greenhouse gas emissions.

NASA notes that “multiple studies published in peer-reviewed scientific journals show that climate-warming trends over the past century are extremely likely due to human activities”.

3. What about natural causes?

Climate does vary naturally — climate change refers to long-term shifts in temperatures and weather patterns — and natural factors such as volcanic eruptions, changes in solar radiation, and natural variations in Earth's orbit can influence climate.

However:

> Climate change is primarily driven by human activities, with greenhouse gas emissions from fossil fuel combustion, deforestation, and industrial processes accounting for the majority of global warming since the Industrial Revolution. Natural factors, such as volcanic eruptions and solar variability, play a minor role in current climate change compared to anthropogenic influences.

When scientists compared possible natural causes (orbital, solar and volcanic activity) over the last 100+ years to possible human causes (aerosols, deforestation and greenhouse gases), greenhouse gases directly correlate to temperature increase.

Current warming is occurring roughly 10 times faster than the average rate of ice-age-recovery warming, and carbon dioxide from human activity is increasing more than 250 times faster than it did from natural sources after the last ice age.

4. What is still debated and what is not?

The basic direction of human influence — that human emissions warm the planet — is considered established fact. As one summary notes, the strongest remaining uncertainties are about regional impacts, feedback strength, and timing, not the basic direction of human influence.

In short: humans are not the only factor that has ever affected climate, but humans are the dominant cause of the rapid warming observed since the mid-20th century.