Many believe that ${\displaystyle CO_{2}}$ and other greenhouse gases (chlorofluorocarbons, methane, sulfur hexafluoride) cause global warming.

• Observed trend in global mean surface temperature
• Observed radiative imbalance at top-of-atmosphere
• Rising atmospheric concentration of ${\displaystyle CO_{2}}$
• Rising sea level due to thermal expansion of sea-water.

Criticism

Some people, for a variety of reasons, claim to have found faults with the hypothesis that humans are affecting Earth's climate. While we strive to present any legitimate criticism of the scientific principles where they are presented, this section includes some specific issues that are commonly cited as reasons that humans are not or could not change the climate.

• Lack of scientific consensus

One of the most common arguments against human induced climate change is a supposed lack of scientific consensus. While there were many skeptical scientists in the past, as the evidence has mounted (especially using satellite-based data), even the most ardent skeptics have come to the determination that humans are changing the climate. One recent study found no instances in the peer-reviewed literature of a study on climate change stating that global warming is either fictitious or purely natural Oreskes, N., Science, 2004, 306(5702), DOI: 10.1126/science.1103618. See also Scientific Consensus.

• The Earth's surface obtains energy from four primary natural sources: space (predominantly solar radiation), the molten core of the Earth, anthropogenic processes that generate excess heat, and radiation from the atmosphere. The second (geothermal heat) is known to be trivially small; the third (direct excess heat) is not as important as increases in the fourth, the energy retention (called the "greenhouse effect"), that is the property of retention heat that could be lost to space due to changes in the atmosphere or surface characteristics, this retention today is mostly due to increases in CO2 levels and receding reflective surfaces like ice or snow. So while indications seems to point to human activities and it is nice to think that changing our energy consumption habits will stop global warming, consideration is needed to account that climate change can be driven by processes that we may have little control over.

• Others, like Bill Ruddiman (U VA) think that we are overdue for an ice age, based on orbital parameters.