We wish to learn:

• What do we mean by "paleoclimate"? 
• What evidence exists for ancient climate change? 
• What is the evidence for recent temperature change?

1. Paleoclimates

Figure 1. Glaciation in North America during the last Ice Age. The geologic record indicates several surges, interspersed with periods of warming, called "interglacials." (Source: Oak Ridge National Laboratory)

From abundant geological evidence, we know that only three hundred and fifty years ago, the world was in the depths of a prolonged cold spell called the "Little Ice Age," which lingered for nearly 500 years. Fifty thousand years ago, in the middle of the last glacial period, large continental ice sheets covered much of North America, Northern Europe, and Northern Asia. Fifty million years ago, global temperatures were so high that there were no large ice sheets at all. 

The speed at which climate can change has also recently become clear: Transitions between fundamentally different climates can occur within only decades. In order to understand these variations, we need to reconstruct them over a wide range of temporal and geographical scales. The importance of this task is underlined by the growing awareness of how profoundly human activity is affecting climate. As with so many other complex systems, the key to predicting the future lies in understanding the past 

We need to ask several questions: What happened? Why did it happen? Has it happened before? Will is happen again? How do we know about it in the first place?

Paleoclimatology

• Isotopic Geochemical Studies: The study of rock isotopic ratios, ice core bubbles, etc. 

 
• Dendochronology: the study of tree rings 

 
• Pollen Distribution: the study of plant types and prevalence 

 
• Lake Varves: (like dendochronology, but with lake sediments) 

 
• Coral Bed Rings 

 
• Fossils: Studies of geological settings, etc. 

 
• Historical documents, paintings, etc. 

Isotope Geochemistry

Oxygen is composed of 8 protons, and its most common form as 8 neutrons, giving it an atomic weight of 16 (O¹⁶) and is also known a "light" oxygen. A small fraction of oxygen atoms have 2 extra neutrons and a resulting atomic weight of 18 (O¹⁸), known as "heavy" oxygen. O¹⁸, is a rare form, with about 1 in 500 atoms of O being heavy. 

The ratio of these two oxygen isotopes has changed over the ages and these changes are a proxy to changing climate in two ways:

Climate Temperature from Ice Cores

Figure 2. Light oxygen in water (H2O16) evaporates more readily that water with heavy oxygen (H2O18). Hence oceans will be relatively rich in O18 when glaciers grow and hold the precipitated O16.

Ice in glaciers has an increased proportional abundance of heavy oxygen if it was deposited during relatively warm periods. To understand why this might be so, we need to think about the process of glacier formation. The water-ice in glaciers originally came from the oceans as vapor, later falling as snow and becoming compacted in ice. When water evaporates, the heavy water (H₂O¹⁸) is left behind and the water vapor is enriched in light water (H₂O¹⁶). This is simply because it is harder for the heavier molecules to overcome the barriers to evaporation. Thus, glaciers are relatively enhanced in O¹⁶, while the oceans are relatively enriched in O¹⁸. This imbalance is more marked for colder climates than for warmer climates. In fact, it has been shown that a decrease of one part per million O¹⁸ in ice reflects a 1.5°C drop in air temperature at the time it originally evaporated from the oceans.

While there are complexities with the analysis, a simple measurement of the isotopic ratio of O¹⁸ in ice cores can be directly related to climate. Ice cores from Greenland are layered, and the layers can be counted to determine age. The heavy oxygen ratio can then be used as a thermometer of old climate.

Climate Temperatures from Ocean Sediments

Many ice cores and sediment cores have been drilled in Greenland, Antarctica and around the world's oceans. These cores are actively studied for information on variations in Earth's climate. 

Figure 3. Changes in temperature as deduced using a number of paleoclimate techniques, each representing different time periods and/or regions.

Other Proxy Indicators

Figure 3 summarizes the climate record as presently understood and lists some of the techniques used for the measurement. 

The most commonly used indicators include pollen, faunal and floral remains, sediment types or composition and geomorphological features indicating physical conditions. In the ocean, indicators such as microplankton, pollen, and sediments settle to the sea floor, where they accumulate to provide a nearly continuous record of climate for millions of years.

The bottom panel shows the record for the last million years. Each successively higher panel expands the shaded part of the panel immediately below. The top panel summarizes the last century.
 

Limitations in Reconstructing Paleoclimates

• Absolute dating

 

 
 
 

Techniques that identify the actual geological time represented by the evidence. Techniques are limited and rely predominately on evaluating the amount of decay of naturally occurring radioactive isotopes.
 

• Relative dating

 

 
 
 

Techniques that are able to differentiate time relative to other points in time. Stratigraphy establishes a relative sequence of events or characteristics within which the evidence lies. If this same sequence can be identified in multiple locations it can be used to establish the relationship between locations and the relative timing of the indicators.

2. Current Climate

Climate records are most often expressed in terms of temperatures, winds, precipitation, and pressures - all parameters that can be measured at multiple sites around the globe. Over the years a large data base of weather event measurements has been obtained, leading to a good description of today's climate.

We find that climate varies widely around the globe - we have deserts and rain forests, ice caps and "death valleys". As for most subjects discussed in this course, there is a taxonomy of sub-disciplines and we can speak of the following:

 

Figure 4.

• Microclimate: climate conditions near the surface over distances of a few meters. Great perturbations to the microclimate can rapidly affect plant life.

 
• Mesoclimate: climate conditions over a few square kilometers, for example, climate of a town, valley, beach, etc.

 
• Macroclimate: climate conditions for a state or a country - over scales ~1000 km or greater.

 
• Global Climate: The overall climate of the planet. We have already discussed, for example, the mean surface temperature and its variations with geologic time. 

Great differences in climate occur from place to place, even within the continental United States which only accounts for about 2% of the Earth's surface. In 1918, a popular climate terminology was developed by Koppen and is called the Koppen System. It is based on annual and monthly average temperature and precipitation measurements, using evidence from vegetation where data is sparse.

Figures 5 uses this system to provide a summary of the types of climates found on today's Earth.

Global Temperatures

Figure 6. Average annual sea level temperatures throughout the world (degrees F)

Note how the isotherms tend to bend along coastlines. This is due to the unequal heating of land and water and the tendency of the winds to blow along coastlines. Also of significance are the ocean currents and the upwelling of cold deep ocean waters. Examples of ocean currents include the California Current which flows southwards along the Californian coast and the Gulf Stream which flows northwards in the Atlantic.

The Atmosphere in Motion

One of the more important factors for climate is the global wind system. Winds are driven into motion by forces on the air. There is a system of prevailing winds whose purpose it is to transfer the excess energy received at low latitudes to high latitudes. If the earth did not rotate and did not have any continental land masses, then the wind system would be rather simple. 

The excess heat received in the equatorial region would cause the air to rise and blow away towards higher latitudes. In order for air to be conserved, the outward motion at high latitudes near the equator has to be balanced by inward low altitude winds. This system is a huge twin-cell circulation pattern (one cell in each atmosphere). These idealized cells are called Hadley Cells. 

Because the earth rotates and has continental land masses, the actual prevailing winds do not directly blow from pole to equator as in but rather curve around and form a multicellular circulation pattern. The curving form the initial direction of the winds is called the "Coriolis effect" and is due to rotation. The curvature is so great as to split up each Hadley cell into three smaller cells.

The highest temperatures occur in the subtropical deserts, e.g., the African Sahara. The lowest mean temperatures occur in Antarctica, where the Sun is either below the horizon or too low in the sky to effectively warm the surface.

It is interesting to note here an important feedback process that can occur at high latitudes. In very cold regions, such as Antarctica, water can only exist as ice and snow. Both these forms of water are very good reflectors of visible light (~80% of visible light gets reflected) and therefore the production of ice and snow tends to depress temperatures by reflecting much of the potentially warming sunlight back to space! This is a positive feedback mechanism - cold temperatures lead to ice - leads to colder temperatures - leads to more ice, etc.
 
 

Global Precipitation

Figure 8. Global patterns of precipitation are closely tied to general circulation and topographic changes 

Current Trends The global average surface temperature has increased by 0.6 ±0.2°C since the late 19th century (IPCC, 2001). It is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record since 1861 (see Figure 8).  As indicated in Figure 9, most of the increase in global temperature since the late 19th century has occurred in two distinct periods: 1910 to 1945 and since 1976. The rate of increase of temperature for both periods is about 0.15°C/decade. Recent warming has been greater over land compared to oceans; the increase in sea surface temperature over the period 1950 to 1993 is about half that of the mean land-surface air temperature. The high global temperature associated with the 1997 to 1998 El Niño event stands out as an extreme event, even taking into account the recent rate of warming.

Figure 9: Combined annual land-surface air and sea surface temperature anomalies (°C) 1861 to 2000, relative to 1961 to 1990. Two standard error uncertainties are shown as bars on the annual number [from IPCC, 2001]

It also appears that the spatial patterns of warming that occurred in the early part of the 20th century were different than those that occurred in the latter part. Figure 10 shows the regional patterns of the warming that have occurred over the full 20th century, as well as for three component time periods. The most recent period of warming (1976 to 1999) has been almost global, but the largest increases in temperature have occurred over the mid- and high latitudes of the continents in the Northern Hemisphere. Year-round cooling is evident in the northwestern North Atlantic and the central North Pacific Oceans, but the North Atlantic cooling trend has recently reversed. 
 

Figure 10. Temperature trends for the periods 1901-1999, 1910-1945, 1946-1975 and 1976-1999. Trends are represented by the area of the circle with red representing increases, blue representing decreases, and green little or no change. [From IPCC, 2001]

4. Summary

• The Earth's climate has changed dramatically in the past, apparently in response to natural changes in orbital characteristics and topography.

 
• We are able to deduce past climates through multiple techniques but much of the progress in resolving Cenozoic climate change has resulted from oxygen and carbon isotope records.

 
• The climate of the Earth today varies by latitude and, to a lesser degree, longitude and is controlled by varying solar radiation availability and the redistribution of energy through wind and currents.