The Earth’s climate has always demonstrated patterns of cooling and warming. Natural factors play a role. However, greenhouse gases (GHG) also influence mean and extreme temperatures, even early in the record. The EU-funded TITAN project examined weather in the 1800s and 1900s, an era that witnessed natural, sometimes extreme events as well as a rise in anthropogenic GHG emission, to better understand and predict climate change.
A look at recent temperature trends
The average global temperature has increased about 0.8 °Celsius since 1880, with two-thirds of the warming occurring since 1975. While this may seem negligible, a one- to two-degree decrease is what plunged the Earth into the Little Ice Age that culminated with extreme cold in the early 1800s. Temperatures then warmed throughout the 1800s, with particularly strong and unexplained warming in the early 1900s. After slowing for a couple decades, temperatures began increasing at an accelerated rate after the 1950s with significantly increasing anthropogenic GHG emissions.
Untangling the factors affecting warming
Volcanic eruptions (and other anthropogenic aerosols) send particles up into the atmosphere that temporarily block the Sun’s warming rays, leading to volcanic cooling. A gap in volcanic eruptions leads to a temporary warming as the climate system moves towards an equilibrium state. According to project coordinator Gabriele Hegerl, “We found from analysing early instrumental and some proxy-based datasets that volcanic eruptions and GHG variations are the key drivers of climate change in the 19th and early 20th century. The early 20th century warming was caused by a combination of GHG increases, warming following a gap in volcanic eruptions, and climate variability. The observed record also contains interesting extreme events, whose risk even early in the record was influenced by humans as well as natural factors.” TITAN’s new sea ice data for the early 20th century better reflect actual climate conditions at the time. As a result, Hegerl explains, “We additionally demonstrated that both climate variability and volcanic eruptions led to deviations from the human-induced warming trend for a number of years. The enigmatic early 20th century warming and record heat waves in the US can now both be explained.” TITAN’s investigations suggest that wet regions get wetter and dry regions get drier with GHG and temperature warming. The completely unprecedented and as yet unsurpassed heat waves in the 1930s in the US were influenced by drought but also by changes in land management, and can be explained with today’s climate models. Climate sensitivity is a metric representing how much the average global surface temperature will change if anthropogenic GHGs in the atmosphere are doubled. An accurate indicator is paramount to policy making. TITAN confirmed the high accuracy of current models and enhanced methods to account for the uncertainty in aerosol effects.
Setting goals for the future
The big question is how much warming is too much. The Intergovernmental Panel on Climate Change (IPCC) aims to limit global warming to 1.5 oC relative to pre-industrial baselines. TITAN found a noticeable GHG influence during the period (1850-1900) used as a baseline by IPCC, suggesting a redefinition of the pre-industrial baseline is warranted. Some of the TITAN consortium is now part of a UK project to predict heat wave changes and their impacts on human health for the future, as well as a project constraining climate sensitivity. TITAN’s detailed and extensive investigations and outcomes provide new insight into factors affecting climate. It is critically needed to address perhaps the most urgent challenge of our time.
TITAN, warming, climate, greenhouse gas (GHG), temperature, Intergovernmental Panel on Climate Change (IPCC), volcanic, anthropogenic, aerosol, ice, cooling, model, uncertainty