Global warming: How do we know the earth is getting warmer?

Take away ideas and understandings:

1. Understand the different data sources indicating the Earth is warming.
2. Know the magnitudes and rates of present global warming signatures.
3. What are the "fingerprints" and "harbingers" of global warming?

• Global warming from historical temperature measurements
• Global warming from borehole temperature measurements
• Global ocean temperatures are rising
• N. Atlantic ocean salinities are falling, whereas tropical oceans are becoming saltier
• Deep Atlantic waters are becoming fresher (thermohaline convection)
• Sea Levels are rising
• Some (not all) alpine glaciers are melting, as are major ice sheets and ice shelves
• Tropical rainfall is increasing.

1.0 Overview: Consensus Versus Certainty in a Complex World.

How do we know the Earth is getting warmer? How sound is the evidence? Is it consistent? Answers to these questions get at the heart of global warming politics. At the core of this debate is the notion of scientific concensus versus certainty and the unease of political action in the face of uncertainty. However there are some things that virtually all scientists can agree upon.

For example, it is 100% certain that carbon dioxide concentrations in the atmosphere are rising and that these increases are due to anthropogenic emissions. It is also nearly certain that surface temperatures are rising globally, on average. The critical question is whether these rises in temperature are due to the increases in greenhouse gases or due to natural climate variability (such as increasing solar irradiance). Other questions which have far less uncertain answers include projections of future amplitudes of global warming and regional climate change.

2.0 Fingerprints and harbingers of global warming.

The "fingerprints" of global warming are those physical properties which document that the planet is, in fact warming. The evidence for recent (last ~100 years) warming of the Earth's surface is compelling. In short, we now have firm evidence documenting the unequivocal warming of surface air temperatures (over land and oceans) and sea level rise.

• Surface air temperatures have risen (globally) near 1.0°C over the last 100 years.
• Earth ground temperatures from boreholes have risen by about 0.7°±0.1°C over the last 100-200 years.
• The ocean temperatures have risen only about 0.1°C over the last 55 years, but the rise is significant.
• Ocean surface salinities at high latitudes are freshening, whereas they are becoming saltier in the tropics. Deep ocean salinities in the North Atlantic are becoming fresher.
• Sea level has risen nearly 12 cm over the last 100 years due to both the melting of ice sheets and the thermal expansion warming surface waters.
• Tropical glaciers have nearly completely melted away, and high-latitude glaciers are broadly retreating (although some are expanding in some regions).
• Invigorated hydrological cycle. A warmer Earth implies significant changes in rainfall patterns, with the equatorial rainbelt becoming more active at the expense of drying in the subtropics and continental interiors.

"Harbingers" of global warming include those physical expressions of climatic extremes which might be anticipated as a consequence of this change but are themselves insufficiently diagnostic to be used as evidence for global warming but they do indicate a consistent direction of change. For example, eight years of the 1990's decade were the warmest years ever recorded since temperatures have been recorded (late 1800's). 1998 and 2001 were the two warmest years on record (here's a global map of the 2001 temperature anomalies). The probabilities of these events happening by chance (i.e not as a consequence of a warming trend) are vanishingly small. Aside from climatic extremes, other factors such as changes in crop growing seasons, animal and bird migration patterns, flood and drought occurrences and durations, and changes in vector-borne diseases are also seen as harbingers of global warming. Examples of global warming fingerprints and harbingers are found here.

3.1 Trends in Global Average Land Surface Temperature.

The figure shows the combined land-surface air and sea surface temperatures (degrees Centigrade) 1861 to 1998, relative to the average temperature between 1961 and 1990. The mean global surface temperature has increased by about 0.3 to 0.6°C since the late 19th century and by about 0.2 to 0.3°C over the last 40 years, which is the period with most reliable data. Recent years have been among the warmest since 1860 – the period for which instrumental records are available.

Warming is evident in both sea surface and land-based surface air temperatures. Urbanization in general and desertification could have contributed only a small fraction of the overall global warming, although urbanization may have been an important influence in some regions. It should also be noted that the warming has not been globally uniform. The recent warming has been greatest between 40°N and 70°N latitude, though some regions such as the North Atlantic Ocean have cooled in the recent decades.

3.2 Trends in Global Average Ocean Surface Temperature.

3.3 Trends in Ocean Surface Salinities

This is a meridional (North-South) cross-section of salinity variations in the Atlantic ocean along 25°W. Note the high salinities (warm color) forAtlantic surface waters and deep waters which flow south, whereas salinities are lower for Antarctic Bottom Waters which flow north. Scientists have been measuring surface and deep ocean salinities for many decades and they have noted that the oceans are becoming fresher, most likely from the addition of meltwater from melting sea ice and glaciers.

Scientists at Wood Hole Oceanographic Institution (WHOI) have discovered that the North Atlantic surface ocean has become markedly fresher over the last 40 years, whereas the tropical oceans have become saltier. Most significant is that the freshening of high latitude oceans appears to be affecting the salinity of deep waters forming in the N. Atlantic. If this trend continues for many decades, the surface will become too fresh to permit subduction of deep water, thereby stopping thermohaline circulation (a very big component of global climate).

Trends in Tropical Ocean Salinities: getting salitier!

Trends in North Atlantic Surface Ocean salinities:getting fresher!

(After Curry et al., 2003)

Trends in deep ocean salinities: getting fresher as well!

from Dickson et al., 2003.

These trends indicate that the Earth's hydrological system is experiencing marked change on a multi-decadal timescale. The changes are consistent with (but do not prove) some global warming scenarios.

3.4 Trends in Land Surface Temperature from Borehole Measurements.

Another way to monitor changes in surface temperature is not to rely on thermometers on land, but to measure the temperature in the land surface itself. Scientists have recently compiled a global set of measurements of temperatures recorded in borehole, holes drilled into rock for groundwater or other purposes. These analyses show that the Earth's surface is warming everywhere and at a rate consistent with the air temperature measurements.

How boreholes can be used to indicate recent temperature changes:

Thermal regime at shallow depths of the crust is controlled by the temperature condition at the surface and the heat flowing from deeper part of the Earth. In an idealized homogeneous crust, if the surface temperature is steady, the distribution of ground temperature is a linear function of depth. However, if the surface temperature changes with time, the ground temperature will depart from the linear distribution which is governed by heat flow (q) and thermal conductivity (k). A progressive cooling at the surface will cool down the rocks near to the surface, increase the thermal gradient at shallow depths, and lead to a temperature profile with curvature like the one shown in green in the illustration above. A progressive warming, on the other hand should be responsible for a temperature profile with smaller even negative thermal gradients at shallower depths like the one shown in red. If the surface temperature oscillates with time, oscillations in the ground temperature will follow.

The magnitude of the departure of ground temperature from its undisturbed steady state is related to the amplitude of the surface temperature variation, and the depth to which disturbances to the steady state temperature can be measured is related to the timing of the original temperature change at the surface. A ground surface temperature history is therefore recorded in the subsurface. By careful analysis of the variation of temperature with depth, one can reconstruct the past fluctuation at the Earth's surface.

Global Average temperature changes from borehole data

How much warming?

Anomolous recent warming or cooling was estimated from borehole at between 400-700 sites globally (Huang et al., 2000; Harris and Chapman, 2001). Most recent results indicate that the surface air temperature has increased globally by about 0.7±0.1°C from the preindustrial period to the 1961-1990 average temperature (Harris and Chapman, 2001).

3.5 Rising Sea Level Trends.

Sea level change is difficult to measure. Relative sea level changes have been derived mainly from tide-gauge data. In the conventional tide-gauge system, the sea level is measured relative to a land-based tide-gauge benchmark. The major problem is that the land experiences vertical movements (e.g. from isostatic effects, neotectonism, and sedimentation), and these get incorporated into the measurements. However, improved methods of filtering out the effects of long-term vertical land movements, as well as a greater reliance on the longest tide-gauge records for estimating trends, have provided greater confidence that the volume of ocean water has indeed been increasing, causing the sea level to rise within the given range.

It is likely that much of the rise in sea level has been related to the concurrent rise in global temperature over the last 100 years. On this time scale, the warming and the consequent thermal expansion of the oceans may account for about 2-7 cm of the observed sea level rise, while the observed retreat of glaciers and ice caps may account for about 2-5 cm. Other factors are more difficult to quantify. The rate of observed sea level rise suggests that there has been a net positive contribution from the huge ice sheets of Greenland and Antarctica, but observations of the ice sheets do not yet allow meaningful quantitative estimates of their separate contributions. The ice sheets remain a major source of uncertainty in accounting for past changes in sea level because of insufficient data about these ice sheets over the last 100 years.

One potentially alarming observation has been the melting back of some (not all) high-latitude glaciers and breakup of large segments of the Antarctic ice sheet, specifically the Larsen ice shelf.

Sea levels are rising because of the dual effects of warming of surface waters (thermal expansion) and because of the addition of glacier meltwater to the oceans.

3.6 Regional Precipitation Trends.

Changes in the strength of the global hydrological cycle can be expected as a result of global warming. Air temperature is a key factor influencing how much water air can carry. In general, the tropics become wetter and the subtropics (north and south) become drier (why is this?); continental interiors also become drier (why is this?). Precipitation has increased over land at high latitudes of the Northern Hemisphere, especially during the cold season. Decrease in precipitation occurred in steps after the 1960s over the subtropics and the tropics from Africa to Indonesia. These changes are consistent with available data analyses of changes in stream flow, lake levels and soil surface. Precipitation averaged over the Earth's land surface increased from the start of the century up to about 1960, but has decreased since about 1980.

3.7 A very interesting example of modern glacier retreat in the tropics: Mt. Kilimanjaro

Scientists at Ohio State University led by Dr. Lonnie Thompson have shown that the "snows of Kilimanjaro" will soon become a thing of the past. By drilling an ice core on this tropical glaciers they have discovered that the ice here is as much as 12,000 years old. Using historical photos of the ice sheet extending back to the early 1900's, they documented the gradual melting of the ice sheet.

The Kilimanjaro ice field has melted by 80% it's maximum size photographed in 1912 AD. Projection of the melting rate into the future suggests that the ice cap will most likely disappear by 2015-2020 (10-15 years from now). This result is important because the ice sheet has been stable at this site for many millennia - the observed recent melting rate is exceptionally rapid.

Thompson, L.G., Mosley-Thompson, E., Davis, M.E., et al. Kilimanjaro Ice Core Records: Evidence of Holocene Climate Change in Tropical Africa. Science, 298, 589-593, 2002.

4.0 Uncertainties in Projections of Human-Caused Climate Warming.

In the realm of global warming science there are things we know, things we believe to be true, and then things which we suspect to be correct but are uncertain. This issue of concensus vs. certainty is so important because political action at the international level requires levels of certainty which may not become evident until the full expression of global warming is underway and perhaps, unalterable.

Highly Certain Facts.

• Humans are increasing greenhouse gases
  
• Greenhouse gases heat the planet
  
• Changed amounts of greenhouse gases have long lasting effects on climate
  
• Other substances (e.g. sulfates, aerosols) cool the planet
  
• CO2 increases and stratospheric ozone decreases have cooled the stratosphere about 1°C
  
• In the last 100 years, the earth's surface has warmed about 0.6°C

Highly Certain Projections (99% chance of being correct).

• As CO2 increase, the stratosphere will continue to cool.
  
• Water vapor will continue to increase in the Earth's lower atmosphere (6% increase for each 1°C of temperature rise.

Probable projections (67% chance of being correct).

• As summer temperatures increase in Northern continents at mid latitudes, soil moisture content will decline
• At high latitude increased precipitation may reduce ocean sainilty, and inhibit the deep water circulation.
  
• As sea surface tempertures rise, some tropical storms may become intense.
  
• As global temperatures rise, there will be an increased probablility of high temperature episodes and a decreased chance of low temperature events.

Highly uncertain projections (direction of change probably correct, projected amplitude unknown).

• The magnitude of global warming for a doubling of CO2 (depends on climate physics, world population and demography, technology, fossil fuel use, etc.)
• The role of aerosols in compensating for greenhouse gas warming.
• Impacts on deep ocean circulation.
• Impact on agriculture, vector-borne diseases, regional climate anomalies.

Adapted from: Mahlman, J.D. 1997. Uncertainities in projections of human-caused climate warming. Science 278: 1416.

Updated April 5, 2004

©2004 P. deMenocal (LDEO, Columbia Univ.)