IPCC Report: Humans Causing Climate Change

The Intergovernmental Panel on Climate Change (IPCC) released the first part of its fifth assessment report (AR5) on Friday.  It clearly states that climate change is real and human activities are the main drivers.

Authored by hundreds of scientists from around the world, the report says, “It is extremely likely (95% confidence) that human influence has been the dominant cause of the observed warming since the mid-twentieth century.” This is stronger language than the panels’ previous report in 2007, which said human activities that emit greenhouse gases were “very likely” (90% confidence) causing the atmosphere to warm. As a consensus report with data gathered from thousands of peer-reviewed published research papers, the wording is very conservative.  Even so, the change of phrasing reflects a significant improvement in scientific understanding and increasing certainty on the issue.

According to the report, global average temperatures have increased 1.5°F between 1880 and 2012. Offering four different scenarios for the future, based on varying controls of greenhouse gas emissions, the panel projects an additional rise in global temperatures ranging from 0.5°F to 8.6°F.  They say the worst impacts of climate change will only be avoided if warming is limited to 3.6°F above pre-industrial levels.  To do this, the report endorses a carbon budget for the atmosphere – an absolute ceiling on greenhouse gas emissions – of one trillion tons.  Globally, we have already released one-half of that amount and energy demand is growing.

The report also found that global average sea level has risen 7.5 inches since 1901. As the world’s oceans thermally expand and land ice continues to melt, the panel expects sea levels to rise another 10 to 38 inches by the end of the century.

Despite the fact that last decade was the warmest on record and extreme weather events are becoming more frequent and intense, climate change still has its skeptics.   One of their fundamental arguments is the decrease in the rate of warming over the past fifteen years. While acknowledging the slowdown, the IPCC emphasizes the fact that short-term records are sensitive to natural variability and do not represent a long-term climate trend. A thirty-year data set is the minimum rule of thumb for dealing with climate and the long-term trend shows overall warming.  In fact, the report states, “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia.”

Formed by the United Nations in 1988, the IPCC assesses climate change and its impacts on society.  It consists of three working groups that publish reports every five to six years.  This most recent document was issued by Working Group I, which focuses on the science of climate change.  Working Groups II and III deal with adaptation and mitigation, respectively. Their reports are due out early next year.  When put together, they form the scientific basis for all U.N. negotiations on global climate treaties.


Graph Credit: IPCC