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 observed 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. Sometimes it seems as if the IPCC starts with the assumption that all recent observed changes in climate (and even extreme weather events) are primarily the result of higher levels of greenhouse gases. However, we know that climates around the Earth have always changed with time regardless of human activity, and it is entirely possible that recent observed changes could be dominated by natural variability. It is one thing to say that observations show that global average air temperature has increased, Arctic sea ice has decreased, sea level has risen, and so forth, but entirely another thing to prove why these changes have happened. The IPCC reports go to great lengths to document observed changes in temperature, precipitation, sea level, and other items, calling them "indicators of climate change"; however, the fact that these changes are observed during a time when greenhouse gas levels are increasing is not proof that the climate changes were mainly caused by human emissions of greenhouse gases and not mainly the result of natural fluctuations in Earth's climate, which have produced large climate fluctuations thoughout the history of the Earth. 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 the global average surface air temperature has increased and that arctic sea ice has decreased, however the fact that these 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 report titled Climate Change 2013: The Physical Science Basis. The full report is 1552 pages.

• 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.
• Here is an interesting article wherein the author claims that the US National Climatic Data Center (NCDC) makes questionable adjustments to past measured temperature data over the United States to give the appearance of a stronger warming trend in recent years. We will not be using this temperature data set in class.
• 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 over large continental-scale regions. (IPCC) 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; however, given that there has been a significant increase in global average temperature, one would expect that there would be more warm temperature records than cold temperature records over this time period.
• At smaller spatial scales (regions within continents), there continues to be much variability when looking at temperature changes in general as well as changes in extreme temperature, with some regions showing significant cooling over the last 50 years.
• 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.
• The 2013 IPCC report concludes that sea level rose at an average rate of 1.7 mm/yr (millimeters per year) from 1901 to 2010, but the rate was higher (3.2 mm/yr) over the period from 1993 to 2010. However, the report also says that the rate of sea level rise was also similarly higher over the period from 1920 to 1950, which corresponds with a period of rising global average temperature from 1910 to 1940 that can not be explained by the small increase in greenhouse gases that happened during that time.
• 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.
• "High confidence" that the ocean surface water is becoming more acidic and that the average pH has decreased by 0.1 since the start of the Industrial Revolution (IPCC). This is a 26% decrease since pH is reported using a log scale.
• There is little doubt that as CO₂ levels increase in the atmosphere, more and more of it will dissolve in the oceans, which makes the water more acidic.
• There is quite a lot of uncertaintly regarding the effects of increased acidity on marine life and the ocean biosphere.
• The pH of ocean surface water varies quite a bit with both location and time of year though, and ranges from 7.8 to 8.4, which seems like a large variation in comparison to the observed decrease in global average pH of 0.1 units.
• Global precipitation patterns have changed. This is something that we would expect with any climate change, whether it is human-caused or natural. Notice from the confidence levels expressed in IPCC reports that it is difficult to determine if there have been significant changes in precipitation based on available observations. And this does not even address the question of whether or not those changes can be directly linked to changes in greenhouse gases.
• "Low confidence" of an increase in global average precipitation (IPCC)
• "Medium confidence" there is no change in tropical precipitation (IPCC)
• "Medium confidence" of an increase in precipitation in northern hemisphere middle latitudes, 30° to 60° North latitude (IPCC)
• "Low confidence" in trend estimates for the rest of the world (IPCC)
• Changes in extreme precipitation (flooding) events and droughts
• "Low to medium confidence" of an increase in extreme precipitation events over large, continental-scale regions (IPCC)
• "Medium to high confidence" of an increase in continental-scale drought severity (IPCC)
• "Low confidence" for a global increase in drought severity, but "high confidence" for certain regions, such as the Mediterannean region and West Africa (IPCC)
• Instructor's note: I question that we have enough data over a long enough period of time to be able to prove or disprove these claims. I noted that this latest IPCC report has lowered their stated confidence for trends in extreme precipitation and droughts. However, based on past variability in these events, we expect there to be shifts in heavy precipitation events and droughts over smaller regional scales regardless of changes in greenhouse gas concentrations.
• Sea ice ... "Very high confidence" that Arctic sea ice has decreased in extent and thickness from 1979 to 2012, particularly in the spring and summer seasons (IPCC) ... "Very likely" that Antarctic sea ice has expanded over the period 1979 to 2012 (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, particularly in the spring months of March through June (IPCC).
• Note that the statement from the IPCC is for springtime snow cover. In the past it was reported that northern hemisphere snow cover was decreasing in all months. Unfortunately many believe that is still true. 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).
• While many 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. This is probably because changes in snow cover are related to temperature, precipitation, and prevailing weather patterns.
• "Very high confidence" that almost all glaciers worldwide have decreased in volume and mass over the period 1979 to 2012 (IPCC). This includes both mountain glaciers as well as the large glaciers in Greenland and Antarctica. In addition, most glaciers will continue to shrink even if the global average air temperature were to stop rising (IPCC). This is because the temperature increases faster than the glaciers are able to melt.
• Melting glaciers would be expected during relatively warm periods. There are many who believe the current melting is similar to previous melting periods. Glaciers grow and shrink on multi-decadal, centennial, and millennial time scales, and recent observations of melting are not outside of the range of natural variability.
• It is worth pointing out that changes in glacial mass are generally slow processes, i.e., it can take a long time for glaciers to grow and a long time for glaciers to melt relative to changes in temperature. Therefore, as glaciers melt away, there will be a lasting impact of future climate, especially for regions located near glaciers.
• "High confidence" that there has been a decrease in both areal extent and thickness of permafrost soils of most regions (IPCC). Permafrost is defined as soil that has been frozen for more than 2 consecutive years. It is stated that the southern limit of the peramafrost soil in the northern hemisphere has been generally moving northward (poleward).
• "Low confidence" that there have been significant changes in small scale extreme weather events, such as severe thunderstorms, tornadoes, and hail in recent decades (IPCC). "Low confidence" that there has been an increase in the strength of tropical cyclones and hurricanes (IPCC). "Low confidence" for an increase in the strength of extratropical storm systems (IPCC).
• As previously pointed out, this is quite different from the 2007 IPCC report, which concluded with higher confidence that these extreme weather events had been increasing. However, this does not stop those in the media from interviewing scientists that are willing to attribute individual extreme weather events to human caused climate change, such as hurricane Katrina, the destructive tornadoes in the United States in 2011, superstorm Sandy, and most recently Typhoon Haiyan.
• 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 increased 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.

On the other hand, there are many climate scientists who do not fully agree with the reports published by government agencies. It is important to realize that there is nowhere near a scientific consensus that fully agrees with the conclusions and predictions of future climate change made within the IPCC report. 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 this short article, Climate Change Impacts In The USA are Already (Not) Happening. A more comprehensive report from the Nongovernmental International Panel on Climate Change (NIPCC), Climate Change Reconsidered II: Physical Science has been updated in 2013. The full report is over 1200 pages and was written by a team of over 50 international climate scientists. There is also a shorter "Summary for Policymakers" similar to that produced by the IPCC. Quoting the first two paragraphs from the NIPCC Summary for Policymakers:

      "The [NIPCC] is an international panel of scientists and and scholars who came together to understand the causes and consequences of climate change. NIPCC has no formal attachment to or sponsorship from any government or governmental agency. It is wholly independent of political pressures and influences and therefore is not predisposed to produce politically motivated conclusions or policy recommendations.
      NIPCC seeks to objectively analyze and interpret data and facts without conforming to any specific agenda. This organizational structure and purpose stand in contrast to those of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), which is government-sponsored, politically motivated, and predisposed to believing that climate change is a problem in need of a U.N. solution."

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