Human impact on climate is a key question — I'll pull the latest scientific evidence to answer it.
Yes — according to the overwhelming scientific evidence and consensus, human activities are the principal cause of the climate change observed since the mid-20th century.
Scientific consensus is unequivocal
•There is unequivocal evidence that Earth is warming at an unprecedented rate. Human activity is the principal cause.
•The Intergovernmental Panel on Climate Change (IPCC) — the United Nations panel of scientific experts — has stated that humans are ‘unequivocally’ responsible for this rapid global warming.
•The findings of their review of more than 14,000 studies are clear: climate change is affecting nearly every part of the planet, and there is no doubt that human activities are the cause.
The certainty has increased with each assessment:
•The IPCC report in 2013 said there is 95 percent certainty that humans have caused most of the warming of the planet's surface that has occurred since the 1950s — up from 90 percent certainty in 2007.
•In its Sixth Assessment Report, the IPCC 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 across the atmosphere, ocean, cryosphere and biosphere.
Multiple independent analyses of the peer-reviewed literature and surveys of climate experts put agreement that humans are causing recent warming at roughly 97% or higher, with some large-scale literature reviews reporting consensus estimates above 99%.
How much warming is human-caused?
•Humans have contributed to 1.07C of the observed warming between 1850-1900 and 2010-19, with a likely range of 0.8-1.3C. As the total observed warming over the same period is 1.06C, this means that humans have caused 100% of the long-term global warming to date.
•Similarly, humans emissions and activities have caused around 100% of the warming observed since 1950, according to the IPCC's fifth assessment report.
Evidence for human mechanism
The mechanism is the human expansion of the "greenhouse effect":
•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.
•Five key greenhouse gases are carbon dioxide, nitrous oxide, methane, chlorofluorocarbons, and water vapor.
Primary human sources:
•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.
Direct observational proof:
•Climate modelling predicts that human activities are causing the release of greenhouse gases and aerosols that are affecting Earth’s energy budget. Now, a NASA study has confirmed these predictions with direct observations for the first time: radiative forcings are increasing due to human actions, affecting the planet’s energy balance and ultimately causing climate change.
•“It’s direct evidence that human activities are causing changes to Earth’s energy budget.”
Why not natural causes like the Sun?
•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.
•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 and lower parts of the atmosphere, because greenhouse gases are slowing heat loss from the lower atmosphere.
In summary, natural climate variability and past climate changes have occurred without humans, but the rapid warming since the mid-20th century — rising surface and ocean temperatures, melting ice, sea-level rise, and increases in greenhouse gas concentrations to levels unprecedented in at least 800,000 years — is attributed by NASA, NOAA, the IPCC and major scientific academies worldwide overwhelmingly to human activities, primarily burning fossil fuels and deforestation.