Recently Observed Climate Change and Implications for Future Climate Changes

Climate has changed in the past and will continue to change in the future. Climate change is a natural part of Earth's history. The issue is whether or not humans are significantly altering the natural progression of climate change, and if so, will these changes be detrimental to life on Earth, including human life? The basic argument made by the IPCC is that human emissions of greenhouse gases has been the major driver for recent observed changes in surface temperature. Quoting from section D.3 of the 2013 IPCC Summary for Policymakers: It is extrmely likely that human influence has been the dominant cause of the obseved warming since the mid-20th century. On this page we will examine the recent observed changes in temperture and climate as well as some of the impacts of those changes. You should realize that changes in temperature will be accompanied by climate changes like changes in sea level and changes in ice cover. These other climate changes really only serve as circumstantial evidence for the IPCC argument because they are expected to happen regardless of why the global average surface temperature may be changing. For example, if the recent warming of global average temperature is simply a natural cycle of climate change on Earth that is not influenced much by higher greenhouse gases, we still expect to see climate changes like reductions in glacial ice. After examining some of the recent observed changes in climate, we will take a look at what could happen in the future if some of the predictions made by current climate models are correct.

Please keep in mind that observed changes do not provide strong evidence for why the changes have happened. Arctic sea ice has decreased in coverage since 1980. We are quite certain of that. However, that observation does not prove that the reason for the decrease in Arctic sea ice is human emissions of greenhouse gases. A recurring message is that correlation does not prove cause and effect. For example, since 1980 we know that greenhouse gas concentrations in the atmosphere have increased and we also know that arctic sea ice has decreased, however the fact that these two events are correlated with each other cannot be used as proof that one caused the other. Unless otherwise noted, the observed changes presented below were taken from the 2013 IPCC reports.

• Global mean surface temperature has increased by about 0.85°C (1.5°F) between 1880 and 2012 (IPCC)
• 17 of the previous 18 years (1995-2012) rank among the 17 warmest years of the instrumental temperature record (since 1850). Instructor's note: Few people argue that global average surface temperature has not risen since 1850. There are questions about how much it has warmed. Uncertainty comes about because adjustments need to be made to measured thermometer records for things like changes in instrumentation, changes in the location of the thermometer with time, changes in population density near the thermometers, etc. Different reasearch groups use different adjustment methods. One also has to consider some of the issues recently brought to light in the climategate scandal, which questions the scientific integrity of some of those responsible for publishing trends in global temperatures. If you are interested in climategate, I suggest you do a web search and read several articles on both sides. You can get started with Watts Up with that Climategate index page.
• Widespread changes in extreme temperture have been observed over the last 50 years. Cold days, cold nights, and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent (see Table SPM-2, IPCC 2007). Instructors note: I have some question about the ability to prove or disprove these conclusions given the limited spatial and temporal coverage of data as well as our limited understanding of the natural variation of these events.
• More than 60% of the net energy increase in the climate system is stored in the upper oceans (0 - 700 meters) and about 30% is stored below 700 meters. Both the surface and deep ocean likely had increases in stored energy since the mid-20th century (IPCC)
• Measurements of changes in ocean heat content are perhaps the most important observations we can make to help understand the impact of added greenhouse gases. If the IPCC climate models are correct, then there should be an imbalance of radiation energy at the surface of the Earth, where more energy is absorbed by the surface than is emitted. This imbalance over the oceans should cause the ocean heat content to increase. According to climate models, this energy or heat should be accumulating in the oceans over time and this is something that we may be able to measure.
• Unfortunately, there is a considerable debate about how the ocean heat content has been changing. There is little doubt that the ocean surface temperature has warmed since 1880 as we have sufficient observations of surface water temperature. However, there is debate about the changes in ocean heat content below the surface based on available observations. A detailed discussion of this issue is beyond the scope of this course.
• A current issue with respect to possible human-caused global warming is that the global average surface temperature has not increased since around the year 2000. Climate models predict that there is an energy imbalance at the surface due to increasing greenhouse gases, so why has the surface stopped warming? A possible answer is that the excess energy is currently being stored in the oceans rather than going into changing the surface temperature. Recent observations of the heat content of the upper ocean (0 - 700 meters) show relatively small changes in this zone, so if this energy is being stored somewhere, it must be in the deep ocean (below 700 meters). Our observations of changes in the energy content of the deep ocean are not good enough to determine if this is really happening. This is because the heat capacity of the deep oceans is so large that it takes an incredible amount of energy to appreciately change the temperature of that much water. It is also difficult to come up with a consensus answer as to physically how that much energy is being moved into the deep oceans. This issue will likely remain unresolved until we are able to make good enough measurements of changes in deep ocean heat content.
• Global sea level has risen 0.17-0.21 meters (6.7-8.3 inches) since 1900. (IPCC)
• Sea level rise is something expected as global temperature increases. Widespread decreases in glaciers and icecaps contribute to sea level rise. Sea level also rises due to thermal expansion of oceans. Thus, warming over an extended period of time almost certainly would result in rising sea level.
• There are many who believe that sea level has been generally rising since the end of the last ice (15 to 18 thousand years ago) with smaller modulations in the rate of sea level rise superimposed due to short term changes in global average temperature. The implication is that changes in sea level are not very sensitive to short term fluctuations in temperature.
• Global precipitation patterns have changed. This is something that we would expect with any climate change, whether it is human-caused or natural.
• Arctic sea ice has decreased in extent, particularly in the spring and summer seasons (IPCC).
  Instructor's note: We have been able to monitor arctic sea ice with satellites since 1979. Over this period there has been a significant decrease in Arctic sea ice coverage, with a record low sea ice area in 2012. Over the same period, though, there has been an increase in Antarctic sea ice coverage, with a record high sea ice area in 2012. Prior to the 1979 and the satellite record, there are accounts of low sea ice periods in the Arctic, e.g., a November 1922 article in Monthly Weather Review, a April 1923 article in the "London Daily News", a April 1939 article in the "Examiner", which talks of an ice-free North Kara Sea and a sea ice line north of 82° latitude. These articles suggest that there are natural cycles of sea ice changes in the Arctic that are not related to human emissions of greenhouse gases. Today's sea ice extent is likely influenced by both natural variation and any temperature changes related to greenhouse gas increases.
  • It is not understood why the Arctic sea ice loss since 1979 has been offset by increases in Antarctic sea ice, such that global sea ice cover has changed little.
• Northern hemisphere snow cover has decreased in recent decades (IPCC). Over the last two decades, Greenland and Antarctic Ice sheets have been losing mass, and glaciers worldwide have continued to decrease in extent. (IPCC) Note that these changes are certainly expected if the temperature is warmer.
  Instructor's note: While Northern Hemisphere snow cover was relatively low in the decades of the 1980s and 1990s, it was certainly not low for the 2001 - 2010 period ( article). Thus, while we expect a general decrease in snow cover with higher temperatures, northern hemisphere snow cover shows large fluctuations that are not related to changes in average global temperature.
• Observational evidence shows that some biological systems have been affected by recent climate changes. These types of adaptations are expected regardless of the reason for climate change.
• Lengthening of middle and high latitude growing seasons
• Poleward and altitudinal shifts of plant and animal ranges
• Earlier flowering of trees, emergence of insects, and egg-laying in birds.

Attributing Observed Climate Changes to Human Activities

The most recent IPCC report (2013) is now more forceful in its statement concerning the anthropogenic influence on observed climate changes: It is extrmely likely that human influence has been the dominant cause of the obseved warming since the mid-20th century. According to the IPCC, extremely likely means over 95% certain. The 2007 report used the term "very likely," which means over 90% certain. Given that the global average temperature has not increases since about 2000, which was not predicted by any of the IPCC climate models, it is somewhat surprising that confidence in the above statement has increased since the last report. The new assessment is said to consider longer and improved records and observations of climate change, as well as improvements in climate model simulations. Instructor's note: I wonder if they are putting too much faith in the ability of climate models.

Beside the International IPCC report, there are reports by US government agencies that claim there are climate change impacts that are already easily observable (e.g., see this EPA page on climate change indicators). In many of these reports, the reader is lead to believe, either explicitly or implicitly, that the climate changes mentioned are largely the result of global warming caused by human additions of greenhouse gases. First, it can be quite difficult to prove that climate has changed significantly given our incomplete knowledge of past climates and our incomplete understanding of how the climate system operates. Then on top of that we would need to prove that the climate changes were significantly influenced by higher levels of greenhouse gases. Most of the climate changes discussed in the EPA reports would be considered negative or bad effects of climate changes and end up being discussed in context of endangered species, forest resources, water availability, disaster planning, and the like. Often these possible bad effects of climate change are given as reasons to guide policy on greenhouse gas emissions in spite of the lack of scientific proof connecting them with increases in greenhouse gases. A skeptical view on some of the recent climate change indicators mentioned by the US EPA and attributing these changes to greenhouse gases is provied in Climate Change Impacts In The USA are Already (Not) Happening.

Model Projections of Future Climate Changes

We should expect that climate will continue to change in the future regardless of whether or not the changes are natural or largely caused by human emissions of greenhouse gases. The following list of items was taken from the 2013 IPCC reports. The models predict continued warming primarily caused by greenhouse gas increases. Because this list is based on climate model projections, these changes are by no means certain to happen.

• Projected increase of global average surface temperature likely in the range 0.8 - 4.8°C by 2100.
• Virtually certain that there will be more frequent hot temperature extremes and fewer cold temperature extremes.
• Projected increase in the rate of sea level rise by 2100.
• Likely increase in precipitation intensity. More heavy rain events and more severe droughts.
• As discussed on previous reading pages, the newest IPCC report has backed off on predictions of significant increases in severe weather events like hurricanes.
• Global oceans will continue to warm. Heat will penetrate to the deep ocean and will affect ocean circulation.
• Reduction in areal coverage of springtime Northern Hemisphere snow cover, decreases in global glacier volume, and continued decreases in Arctic sea ice.
• Increases in Ocean acidification. This is certain with higher concentrations of CO₂ in the atmosphere. However, there are arguments about how much ocean pH will change, which are related to the buffering ability of the oceans, and how this will affect ocean life.
• Disruptive changes in biological systems in response to climate changes.

Potential Impacts of Climate Change / Impact Studies

Impact studies are done to try to understand how humans and other life will be affected by climate changes. This is really the ultimate question to answer. Unfortunately, we are not very good at predicting future climate changes, which makes impact studies uncertain. Most impact studies today are performed by models that first read as input the climate changes predicted by a climate model. Thus, if climate model projections for the future climate in a given ecosystem are wrong, then the projected impacts on that ecosystem will be wrong. Given the uncertainty in climate model projections, it is probably a good idea to perform impact studies on a whole range of possible future climates before deciding any long-term policies. Also keep in mind that even if we know for sure how the climate will change within a given ecosystem, it is still difficult to understand and predict how life within that ecosystem will adapt. Below is a list of some of the considerations within impact studies:

• Human health, natural ecological systems, and socioeconomic systems are all sensitive to both the magnitude and the rate of climate change
• Many physical and ecological systems will be simultaneously affected
• The ability of natural ecological systems to migrate appears to be much slower than the predicted rate of climate change estimated by climate models
• Climate change can add to existing environmental stresses, some of which are brought on by other human activities, such as deforesting a region for human settlement

Please also read over this WORD document defining Impact Studies and Outlining possible consequnces of Global Warming. The figures and links below are from the EPA's pages on the impacts of climate change. As you might expect, this government website is biased toward possible negative impacts of climate changes resulting from human emissions of greenhouse gases.

Health Impacts	Water Resources Impacts	Forest Impacts
Coastal Areas	Species and Natural Areas	Agriculture Impacts