The Intergovernmental Panel on Climate Change issued its full report on Monday, detailing the latest findings, assessments, and predictions about our current global climate system and how it will change in the future. This is the fifth such report from the IPCC, a colossal joint effort between scientists specializing in a broad range of disciplines, all sharing their knowledge, research and data to make this collaboration possible.
You might have seen the news that, in the latest report, the IPCC has increased its confidence to 95% (up from 90% in its last report) that humans are having an impact on the global climate. Before I delve into any findings in the report, it should be mentioned that scientists are famous (or notorious) for not using definite statements. As a meteorologist, for instance, I abhor saying that there's a 100% chance or 0% chance of something happening in the weather. In this IPCC report, the scientists refer to the level of confidence they have with phrases such as "virtually certain" and "medium confidence". All of the confidence-defining words and phrases are defined within percentage ranges. Suffice it to say, they take this stuff very seriously. In this article, I italicized the confidence terms used in the report, just like they did.
Of course, when trying to make statements of confidence, these scientists were up against some big barriers. Weather records only extend back as far as a city, airport, or weather observation station has been in place. Gathering climate data from before weather instruments were widely used, or even before they were invented, can be tricky. But there is "paleo-evidence", if you will, in the form of ice core samples from glaciers, data from tree rings, rock layers in the geological record, and other data sources. So, we can't know what the high temperature was in Washington DC on today's date 100,000 years ago, but we can tell what the gas composition was of the atmosphere, and what type of plant and animal life existed. Scientific data is advanced enough today that we can use these pieces of evidence to see what the climate was like, dating back hundreds of millions of years. It's this inter-disciplinary collaboration within the scientific community that makes this report possible.
Most of the findings can be summed up as a strengthening of confidence in the conclusions that were previously drawn in earlier IPCC Reports. I'll focus on the findings for our part of the world- the midlatitudes of the Northern Hemisphere. In the report, the Panel talks about the long-term cooling trend, which had been in progress for about 5,000 years in the Northern Hemisphere. The report says, with high confidence, that this cooling trend has been reversed in the past 50 years or so, in the mid-to-high northern latitudes (places like the USA, Canada, Russia, and Europe). This is the part of the globe which has the most land mass, which may be part of the reason that the trend reversal is so evident here. But they also say in the report that the data collection is the most thorough in this part of the world, adding to their confidence in the data and findings.
Another big conclusion from the report is that it is likely that atmospheric circulations have shifted poleward since the 1970s. What does this mean? Circulations, such as the jet stream, are now traveling at a higher latitude on average than they did 40 years ago. The jet stream serves as a superhighway for storm systems, so a different storm track will bring more precipitation to areas that are accustomed to drier weather, and vice versa. Since the jet stream also divides airmasses of different temperature and moisture levels, a shift in its pattern could affect growing seasons, impact water usage, and steer tropical systems in abnormal paths.
The report also concludes with very high confidence that glaciers and ice sheets around the world are continuing to shrink. And, in one of those weird quirks of thermodynamics, the ice hasn't been able to melt as fast as the temperature has changed. This means that, even if Earth's average temperature were to suddenly become static, the ice would continue melting.
Scientists from 39 countries around the world were involved in the IPCC report in some way. On Monday, five of the key players took part in a Google hangout to provide their insights. They said that the increased confidence in human-influenced climate change is mainly because of better technology for assessing ice sheets at the Earth's poles, and more advanced computer modeling. According to Eric Rignot of the Jet Propulsion Laboratory, the report represents "multiple lines of research converging on the same answer", resulting in such a high level of confidence. As far as the short-term effects are concerned, Dr. Ralph Kahn of NASA Goddard said that "wet places are likely to get wetter, and the dry places are likely to get a bit drier".
It's very important to remember that our climate has always changed. You've heard of the Ice Ages, right? And when dinosaurs roamed Earth and grew to unfathomable sizes, our planet was in the warmest pattern of its history. However, today's global climate is influenced by human activity. The scientific community at large is virtually certain of it.
This blog post really just scratches the surface; there's a ton of data assessments and findings in the report. If you'd like to check out the research for yourself, the technical summary is available here (warning: It's LONG, and it's very scientific). Or, you can check out the IPCC's page.