Authored By: 

Katherine Bagley and Zahra Hirji

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The Intergovernmental Panel on Climate Change, part of the United Nations and the world's largest scientific body on global warming, on Friday released the first portion of its long-awaited assessment on the causes and consequences of climate change. The report took six years to produce and is considered the most comprehensive review of the scientific evidence for man-made climate change available. It is the fifth such report since 1990.

The report's main conclusion, summarized by scientists and approved by government officials from more than 120 countries, is that human-caused "warming of the climate system is unequivocal," with many of the impacts of warming already "unprecedented over decades to millennia."

The following slideshow by Katherine Bagley and Zahra Hirji of InsideClimate News is your guide to the top 10 things you should know about this latest landmark report.

Credit: IPCC 2007

The Process, by the Numbers

More than 830 scientists from 85 countries contributed to the IPCC's fifth assessment. Over the course of six years, IPCC authors reviewed 9,200 scientific studies shedding light on topics ranging from observations of rising sea levels and cloud formation to simulating the climate's sensitivity to the atmospheric build-up of heat-trapping greenhouse gases. They also received more than 136,000 comments from experts who reviewed drafts of the research.

Credit: Mat McDermott, Flickr

How Much More Certain Can It Get?

Twelve years ago in its third assessment, the IPCC asserted with 66 percent certainty that greenhouse gases from human activities were causing the majority of observed global warming since the early 1950s. In 2007, the panel increased its confidence level to 90 percent.

On Friday, the IPCC raised that certainty even more, this time to 95 percent. In IPCC terms, this means it is "extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

Credit: Johannes Frandsen

A Carbon Budget, and We're Halfway There

For the first time, the IPCC warned that the world needs to stick to a "carbon budget," and it specified the levels of future emissions that are allowable in order to have a reasonable chance of staying within 2 degrees Celsius (3.6 degrees Fahrenheit) of warming, the safe climate target.

In short, humans can only burn about 1 trillion tons of carbon before 3.6 degrees Fahrenheit of warming becomes inevitable. Nations have already consumed over half of that amount since the beginning of the Industrial Revolution.

By using the "carbon budget" concept, the IPCC endorsed the gist of the Potsdam Institute for Climate Impact study of 2009, popularized by climate activist Bill McKibben's Rolling Stone "Do the Math" article.

Credit: Se├▒or Codo, Flickr

A Pause in Temperature Rise. So What?

The question of whether the rise in global temperatures has "paused" in recent years has been hotly debated in recent months, mainly within the media. Skeptics of climate science have latched on to this observation as evidence that scientists are overstating the evidence for man-made global warming. In its fifth assessment, the IPCC says that is not the case.

The scientists do acknowledge that global average surface temperatures haven't significantly risen in the last 15 years or so. But they are careful to point out that the plateau doesn't mean that climate change has slowed down or stopped as a result. They claim that approximately half of the pause is due to more heat being trapped within the deep ocean. (That is heat that otherwise would be felt in the atmosphere; therefore, it's skewing the surface temperature records.)

Credit: NOAA

More on That Stubborn Pause

The report says that the other half of this "pause" can be attributed to the cooling effect of volcanoes and a less active sun, leading to less-than-normal warming. Scientists credit better climate models for being able to make these distinctions, though there is still fierce debate over the exact role of these factors in the so-called pause.

Further, the IPCC says that while surface temperatures may not have significantly increased in recent years, the impact of climate change is evident and growing in many other places across the world, such as in melting glaciers and sea ice.

Credit: Kimon Berlin, Flickr

Better Models: What a Difference

Since the last IPCC assessment came out in 2007, more of the historical climate record has been analyzed; scientists generally understand the Earth's climate system better; and computer systems have become more advanced. All of these factors contributed to the development of an improved climate model for the latest IPCC assessment. The model, known as CMIP5, formulates projections for things like sea level rise and drought in more detail and on a smaller scale than ever before. It also handles localized features better, such as cloud formation and rainfall.

Work on the model began in 2008 through a collaboration of 20 climate modeling groups at top research universities and institutions around the world.

Credit: NOAA

Argo Unlocks Ocean's Mysteries

Since 2000, scientists have deployed more than 3,600 floating sensors from the North Pole to the South Pole to help monitor and better understand the world's oceans. The probes, known as the Argo network, measure temperature and salinity down to a depth of 1.2 miles. These observations and research have dramatically increased scientists' understanding of how the oceans are coping with climate change, and they are incorporated in the latest IPCC assessment. For example, scientists now have much better data for how quickly the oceans are warming. Since water expands as it warms, these findings are key factors in the IPCC's sea level rise projections.

Credit: NOAA

World's Coastlines to Bear the Brunt

In 2007, scientists were unclear exactly how the Greenland ice sheet would react to a rapidly warming world, a factor that many critics used to question the sea level projections in the IPCC's fourth assessment. This time around, scientists say they have a lot more figured out. As a result, they have increased their sea level rise projections.

The IPCC now says that the world's oceans could rise anywhere from 10 inches to 2.7 feet by the end of the 21st century, depending on the increase in greenhouse gas emissions. The rate at which this rise will occur is unclear, but the scientists estimate that about 70 percent of the world's coastlines will be affected.

Credit: Baldeaglebuff, Flickr

More Deadly Weather to Come

Countries across the globe have been slammed by extreme weather events in the recent years. Within the past 12 months in the United States, Hurricane Sandy, record-breaking wildfires, devastating floods and serious drought took dozens of lives and cost billions of dollars in damage. According to the latest IPCC report, this trend is only expected to get worse in coming years.

Extreme precipitation events will become "more intense and more frequent" by the end of the 21st century. Monsoons will grow in size and last longer. Droughts will worsen and stretch for longer periods of time. The most intense hurricanes will happen more often, particularly in the North Atlantic and Western Pacific.

Credit: Robert Simmon, NASA Earth Observatory

The Heat Is On

In its 2007 report, the IPCC concluded that the world could warm by between 3.6 and 8.1 degrees Fahrenheit by the end of the 21st century if carbon dioxide levels continue to increase. The latest report drops the bottom end of this estimate slightly, stating that temperatures could rise between 2.7 and 8.1 degrees Fahrenheit.

The scientists also said it is "very likely that heat waves will occur with a higher frequency and duration."

Credit: Vinod Panicker, Wikimedia

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