Human activity is the main cause of warming in recent decades
From the analysis of gas bubbles trapped inside polar ice samples, we know that the atmospheric CO₂ concentration in the pre-industrial era (before 1750) was 280 ppm. The IPCC's fifth report, in 2011, reported a current concentration of 430 ppm. Most of the increase (over 70%) has occurred since 1970. The Russian-US-French programme at the Vostok station in East Antarctica, which studies the deepest ice samples recovered to date, has been used to reconstruct the situation over the last 400,000 years: the current situation has no precedent in that period.
The scientific community is studying the causes—both natural and anthropogenic—that may lead to heating or cooling of the Earth: anthropogenic factors that contribute to warming include the increase in greenhouse gases and ground-level ozone, but there are also cooling effects produced by human activity, such as aerosols and increased surface albedo caused by changes in land use. Moreover, variations in solar activity are an important natural source of warming, and volcanic eruptions are a natural factor contributing to cooling; both are also being examined.
In its latest report (AR5, 2014), the IPCC increased the degree of scientific certainty that human activity is responsible for the warming that the world has experienced, from "very likely" (i.e. a confidence level of 90%) in 2007, to "extremely likely" (a confidence level of 95%) at present.This report compiled all the natural and anthropogenic factors leading to changes in energy flows that shape the climate (CO₂ and CH4 emissions, but also particle emissions from volcanic eruptions and changes in solar radiation), and concludes that the largest source of changes to the climate system is the activity of humans: the release of CO₂ into the atmosphere.
Scientific knowledge continues to advance and, in the next few years, complex simulation models are likely to become much more sophisticated, enabling us to assess variables that determine the speed of the process, which is the main element of uncertainty at this time.
Guest post written by Carmen Becerril Martínez, External Director from ACCIONA, and Magdalena García Mora, Manager of Analysis of Energy policies and Climate Change from ACCIONA.