Unprecedented Climate Change May Hit the Tropics Much Sooner Than Expected


October 2013

There are many ways of describing the impact of climate change on ecosystems and human societies around the world. But sometimes it takes a fresh set of eyes to summarize complex data sets in new and creative ways.

That is what a class of graduate students at the University of Hawaii in Manoa did under the direction of Dr. Camilo Mora. Mora asked his students to take a closer look at forecasts produced by 39 scientifically validated climate models whose results are publicly available. These models are computer programs that simulate the climate’s response to current and future greenhouse gas emissions based on the known physical and chemical properties of the Earth’s atmosphere.

Manokwari, Indonesia, is one of the places facing a "climate departure" within a few decades
Manokwari, Indonesia, is one of the places facing a “climate departure” within a few decades

The models were developed by 21 research teams from 12 different countries; they predict a range of plausible future climate scenarios with a well-defined level of statistical uncertainty, or margin of error. Many research papers had already been published on the basis of these 39 climate models before Mora designed his class project.

The novel contribution of Mora and his graduate students, which earned them a high-profile article in Nature, one of the world’s most prestigious scientific journals, was to come up with a new and meaningful summary of the 39 different sets of results. They called it the “climate departure date:” for any geographical location in the world, it describes the year after which all future years are expected to have a higher average temperature than the warmest year in that location’s historical record. 

Across the globe, this historical record could be constructed for the past century and a half, from 1860 to 2005. When the students averaged the results from all 39 climate models, they estimated the climate departure date for more than half of our planet to be as early as 2047; in other words, even the coldest year after 2047 is expected to be warmer than the hottest year between 1860 and 2005 in the majority of the world.

Factoring in the global margin of error, this shift may happen anywhere between 2033 and 2061, with 2047 being in the middle of that range. This estimate assumed that the burning of fossil fuels would continue to increase as it has in the past. If, instead, vigorous global efforts were to stabilize greenhouse gas emissions at their current levels, the estimated climate departure date would only be pushed back by 20 to 25 years.

Since the climate departure date can be calculated for any specific geographic location, the students were able to demonstrate that tropical regions will likely experience a significant departure from their historical records much sooner than countries in northern latitudes with higher climate variability. On average, Mola noted in a press conference, the tropics will experience unprecedented climate change some 16 years earlier than the rest of the world.

For example, climate departure for New York City is expected to happen in 2047 under the high-emission scenario, and in 2072 under the low-emission scenario; these local estimates have a 5-year margin of error, much smaller than that for the global estimates. The high-emission estimates for tropical regions were 2020 for Manokwari, Indonesia, 2029 for Lagos, Nigeria, and 2033 for Bogotá, Colombia.

The earlier expected changes in the tropics are particularly worrisome for two reasons: this is the home of the world’s greatest biological species diversity, and of billions of people, many of whom live in economically poorer countries.

Since the ability of tropical animals to either move to a more suitable climate or stay put and adapt is quite limited, the danger of species extinction is high. As for the human impact, co-author and graduate student Ryan Longman said that between 1 and 5 billion people will live in areas with climate departure dates before 2050.

“Countries first impacted by unprecedented climate change are the ones with the least economic capacity to respond. Ironically, these are the countries that are least responsible for climate change in the first place,” Longman said in a telephone interview.

Click here to read study reports by NBC News and the New York Times.

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