Scientists have historically attributed climate change to small fluctuations in the earth's orbit, which affects the levels of solar energy absorbed and retained by the planet's land, oceans, and atmosphere. The level of absorption and retention is affected by the composition of the atmosphere. A dramatic and highly unprecedented (in the last 400,000 years) spike in atmospheric carbon dioxide levels, from roughly 300 parts per million in 1950 to approximately 400 parts per million today, is mostly attributed to human activity. This activity is associated with the industrial revolution, especially industrialization since the mid 20th century, at a probability rate of 95% (according to the Intergovernmental Panel on Climate Change's Fifth Assessment Report). The data is based on ice core samples and other paleoclimate measures combined with modern direct measurements.
Physicists since the 19th century have recognized that changes in atmospheric composition, including a rise in heat-trapping carbon dioxide, can raise atmospheric temperatures. This is the well-known greenhouse effect. Measurements from finely calibrated instruments taking readings from earth-orbiting satellites, weather balloons, and surface instruments have been carefully compiled and demonstrate that more infrared energy from the sun is being captured and stored in the atmosphere, causing the planet's average surface temperature to rise by 1.62 degrees Fahrenheit since the late 19th century. Scientists mostly attribute this warming to increased carbon dioxide and other anthropogenic gas emissions, especially in the last 35 years, with the most rapid warming occurring since 2010.
NASA reports that 97% of climate scientists agree that these climate-warming trends over the past century are most likely due to human activities related to the industrial revolution. Ice core samples from around the world clearly indicate a correlation between carbon dioxide levels and the earth's atmospheric temperature. Paleoclimate evidence compiled from seabed samples, sedimentary rock, tree rings, and coral reefs also support the contention that the current rate of warming is roughly ten times faster than the average rate of recovery from ice age temperatures (according to the National Research Council (NRC), "Surface Temperature Reconstructions For the Last 2,000 Years". National Academy Press, Washington, D.C. 2006).
Most of this rise in temperature has been absorbed by the earth's oceans, with the upper 2,300 feet or so warming by at least 0.4 degrees Fahrenheit since 1969, leaving the greater part of the warming to the atmosphere (according to NOAA, National Centers for Environmental Information).