Why is 2ºC the Limit for Warming?

Written By Teacher: Teresa Pettitt-Kenney

Hi there! My name is Teresa and I just finished my Bachelor's degree in Environmental Science and am excited to pursue environmental education in the future! I am extremely passionate about climate change, equitable climate action, and how education can work to address these issues. 

Understanding the science behind the 2ºC warming limit can empower you and your students to confidently discuss climate change and its impacts. This infamous number represents a tipping point for dramatic shifts in Earth’s systems. Explore the history and science behind this limit in order to help students grasp the real-world implications of global warming and engage in meaningful conversations about solutions. Watch this short TED video, Why Is 1.5 Degrees Such a Big Deal?, to start the discussion and empower students to take action with videos of their peers advocating for change: Greta Thunberg Speech at COP25. 

Written By: MIT Environmental Solutions Initiative

The MIT Climate Change Engagement Program, a part of MIT Climate HQ, provides the public with nonpartisan, easy-to-understand, and scientifically-grounded information on climate change and its solutions.

The Intergovernmental Panel on Climate Change (IPCC) was founded in 1988 to assess science related to climate change on behalf of the United Nations. While its reports summarizing climate science from around the world are influential in international diplomacy, it was not the first body to note the importance of limiting global warming to 2° C.

In the 1970s, William Nordhaus, an economist at Yale, suggested in several papers that if global warming were to exceed 2° C on average, it would push global conditions past any point that any human civilization had experienced. At the time, Nordhaus's idea was a simple suggestion of what rise in temperature could cause extreme conditions, based on the historical record of past average temperatures, but it gained new importance a decade later.

In 1988, amid mounting evidence that the earth was warming, James Hansen, a NASA scientist, testified before Congress and became one of the first scientists to publicly link greenhouse gas emissions from humans to this warming trend. Hansen warned that if the world did not reduce emissions, it could result in catastrophic climate change, causing sea-level rise, extreme weather and damage to ecosystems and human settlements across the globe.

"At two degrees we see dramatic alterations to the ability of the Earth's system to maintain the conditions that allow for human life and indeed other species’ life," says Maria Ivanova, a professor of global governance at University of Massachusetts Boston.