How likely global warming is human-caused? 95% says new IPCC report

Earlier today (September 27, 2013), the Intergovernmental Panel on Climate Change (IPCC) released its 5th assessment report on the state of the Earth’s climate. The goal of this assessment is to lay out a scientific foundation for governments around the globe with respect to policiies related climate change. Is it even possible to slow down climate warming over the next several decades? The report does not consist of new research, but it does consist of hundreds of climatologists (250 authors from 39 countries) reviewing past studies and writing a substantive report about the state of the climate. Do they all agree perfectly with each other? No. That would be impossible. Still, in the IPCC’s 5th assessment, overall confidence that climate warming is human-induced is placed at 95%.

The IPCC uses words such as “likely” to express scientific certainty (or lack of certainty) on specific topics.

For instance, in the 2001 assessment, the IPCC were less than sure, and expressed that human-induced global warming was a greater than 66% probability. By the 2007 report, they were more sure, using the phrase “very likely” that human-induced warming is occurring and assigning it a 90% probability.

Now, in 2013, they are at 95% confidence.

The report specifically states:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased. Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

Sea level change over the past century. Image Credit: IPCC
Sea level change over the past century. Image Credit: IPCC

Global Oceans: Warming and Rising

In the report, the IPCC states that it is extremely confident (95%) that ocean temperatures are warming, especially since 1950, and that the majority of the energy stored across the globe is being sent to our oceans. The report specifically states that sea level rise will continue, as the Earth continues to warm. The new IPCC report suggests that sea level will continue to climb by at least an extra 0.9 – 3.0 feet (26 to 90 cm) by 2100. The projected numbers are only based on the currently sea level rise and do not take into account in additional melting of landmasses such as Greenland. With this in mind, these numbers are thought to be conservative based on what the majority of climate scientists believe.

Arctic summer sea ice melt over the past century. Image Credit: IPCC
Arctic summer sea ice melt over the past century. Image Credit: IPCC

Earth’s Cryosphere: Ice continues to melt

Over the past two decades, Greenland and Antarctica have both been losing mass in the form of melted ice. For Antarctica, there is very high confidence this occurred in the mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica. Glaciers are retreating, and the rate of ice melt in the Arctic is on a trend indicating that ice will continue to melt at larger rates over the next several decades. There is a chance that the Arctic could become ice free by the middle of this century. Snow extent continues to lessen across the Northern Hemisphere, and is expected to decrease at a rate of 7% to 25% by the end of the century.

Global temperatures have continued to rise over the past century. Image Credit: NCDC
Global temperatures have continued to rise over the past century. Image Credit: NCDC

Explanation of the “slow” warming over the past 15 years

Climate is complicated and ever-changing. To observe Earth’s climate change, you have to look at temperatures across the globe over decades. Temperatures continue to rise each decade, and the time span from 2001-2010 shows that it was the warmest decade we have observed.

However, since 1998, the rate of change has been slower. In other words, temperatures are not climbing as fast as they were, in the previous decades. Are up-and-down variations from year to year – and even decade to decade – to be expected? Yes. There will be upswings, and downswings.

Can we draw an illustration, using your own human body? Let’s say you were diagnosed with a disease and you had only three months to live. Your overall health would eventually decline, but how you feel from day-to-day, or week-to-week would fluctuate. You might feel great for three days of the week, but the other four could be rough. When it comes to observing the climate system, this analogy works. For instance, sea ice extent was larger in 2013 than in 2012. However, 2012 was a rare year in which we saw the lowest Arctic sea ice extent since 1979. When you see such an extreme low, it can be expected that the following year will likely have higher sea ice extent.

Another example from real climate came in 1998, a rare year in which we saw an extremely strong El Niño form. When El Niño forms, you typically see an uptick in global temperatures. The next time we see a strong El Niño form, we could easily see global temperatures climb even more and at record levels. It will happen. The only uncertainty is when and how strong an El Niño will form.

So fluctuations or changes from year to year, or even decade to decade, are expected. What’s critical is that the trend continues to show an increase in ice melt, caused by rising temperatures.

Hurricane Sandy on October 28, 2012 via the GOES-13 weather satellite. As of now, we are unsure how climate change affects tropical cyclones.
Hurricane Sandy on October 28, 2012 via the GOES-13 weather satellite. As of now, we are unsure how climate change affects tropical cyclones.

Uncertainties about future weather events

When it comes to extreme weather events, there are still plenty of uncertainties that climate scientists are trying to understand.

For instance, there is low confidence regarding possible increased intensity of tropical cyclones around the globe. Confidence also remains low for increased intensity and/or duration of drought.

Meanwhile, confidence is growing that warmer days and nights will continue to increase, and we will continue to see fewer cold extremes. There is high confidence since 1950 that precipitation has increased in the mid-latitude land areas of the Northern Hemisphere. Unfortunately, extreme precipitation events appear very likely towards the end of the century.

Bottom line: The Intergovernmental Panel on Climate Change (IPCC) released the Summary for Policymakers earlier this morning (September 27, 2013) stating that confidence of human-induced global warming of Earth is now at 95%, or extremely likely. An increase in sea level rise, carbon emissions, sea ice melt, and global temperatures are to be expected and might cause significant impacts to the way we live. Three more IPCC reports will be released, describing what we can do to help slow down this process and more.

September 27, 2013

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