Analysis: Recent Global Warmth Is Natural, Benefits Humans

Published July 1, 2005

The current ice age, called the Pleistocene, became severe approximately two million years ago. One trait of the ice age is its pattern of glacial and interglacial periods, the harsh and cold glacial period persisting roughly 100,000 years, followed by a moderate interglacial period lasting only approximately 10,000 to 15,000 years.

Around 10,000 years ago, the cold abated and marked the onset of the present interglacial period, called the Holocene, as massive ice sheets at middle to high latitudes shrank, subsequently raising sea levels and inundating the extended, continental boundaries previously defined by the glacial conditions.

The next glacial is expected to begin within several millennia.

Warming Predictions Unrealistic

Discussions on enacting caps like the Kyoto Protocol on carbon dioxide emissions arise from forecasts by computer simulations of climate conditions centuries in the future. Simulations contain substantive uncertainties and unknowns and are essential scholarly tools; they cannot accurately reproduce major features of climate. Indeed, measurements and analyses of relevant climate parameters suggest so far a much smaller enhanced greenhouse effect than the computer simulations do.

While there was a warming trend in the last decades of the early twentieth century, coinciding with and possibly caused at least in part by the enhanced greenhouse effect, there was a prior warming trend of equal magnitude early in the twentieth century apparently not primarily caused by the enhanced greenhouse effect.

If the recent warming trend, observed to be roughly 0.15-0.17º C per decade, is assumed to be caused entirely by the enhanced greenhouse effect, it is somewhat lower than projections from most computer simulations, indicating the forecasts are still uncertain.

Baseline May Be Wrong

Regarding natural climate variability, it should be noted that the nineteenth century was the end of a well-documented, centuries-long cold period in many areas of the world. Hence, the period of unusual cold at the start of the instrumental record may bias the casual observer to believe the second half of the nineteenth century displayed “normal” temperature, and the twentieth century is “abnormal” in warmth.

One insightful test of estimates of the enhanced greenhouse effect comes from predictions made by the computer simulations of air temperature just above the surface. Simulations forecast increased temperature from the surface to a height of several kilometers (km) ( and The air at those altitudes should already display an accelerated warming trend with respect to the surface if simulations of the enhanced greenhouse effect are correct (

Measurements from weather balloons since the 1950s and NOAA satellites beginning in 1978 have yielded an independently validated record of temperature integrated over the layer from the surface and rising to approximately 5 km, or the low troposphere. (See Figure 1.)


Figure 1 – Monthly averaged change in temperatures for the layer of air from approximately the surface to a height of 8 km above the surface through June 2004 as measured by a series of NOAA satellites over most of the globe ( and verified by good, independent balloon measurements ( Simulations of the enhanced greenhouse effect forecast a warming trend of approximately 0.25 to 0.35º C per decade, or accelerated warming compared to the surface. The well-validated temperature of the low troposphere shows a significantly smaller trend, +0.077º C per decade, through September 2004, which is several months more recent than the chart (

A linear trend fitted through the record, +0.077º C per decade, is smaller than that at the surface, in contradiction to the climate simulations of the enhanced greenhouse effect. It suggests the incomplete simulations are predicting overly high warming trends, both for recently past and future decades.

Recent Warmth Providing Stability

Is the twentieth century’s climate unusual? This perspective from ice core information covers the last 17,000 years and pertains to Greenland (see Figure 2), although features of the record are present in other regions. The record begins around the time of the coldest recent period of the glacial. During the termination of the glacial period, temperatures fluctuated sharply, and the physical details of causal factors and climate responses are poorly known.


Figure 2 – Temperature and ice accumulation over the past 17,000 years, with climate and societal features noted, as developed from ice cores taken sampled in Greenland (courtesy Dr. Art Green). Note: temperature refers to the mean temperature of Greenland.

Response by humans and ecosystems to the retreat of the glacial period and onset of a more stable and warm climate was swift. With the development of agriculture, human civilization expanded and sculpted extensive, artificial landscapes.

Compared to so great a change of the glacial termination, the last one thousand years look fairly calm. But significant fluctuations in local conditions did occur, driving notable ecosystem and human responses.

Humans Struggle in Cooler World

A broad period of equable climate reached parts of Western Europe as early as the ninth century C.E. and persisted in some areas though the twelfth century. Peoples in Western Europe could grow familiar crops at more northerly latitudes or higher altitudes than had been possible in prior centuries.

By the twelfth to thirteenth centuries, a series of harsher periods set in, some appearing seemingly abruptly. Economies had benefited from agriculture and sea trade; the onset of the climate deterioration eroded economies and shocked cultures. Called the Little Ice Age, it persisted in areas of Western Europe into the nineteenth century.

Life expectancy in England, which had gained approximately 10 years during the Medieval Warm Period, fell back to roughly 38 years, according to climatologist Hubert Lamb, by the mid-fourteenth century.

Unusual weather calamities continued to strike in the fourteenth and fifteenth centuries. One diarist noted that in Smolensk (Central Europe) in 1438 the starvation was so bad “the wild animals ate people and people ate people and small children.” Survival meant cannibalism.

Recent Climate Unexceptional

In terms of extreme weather, the twentieth century’s storminess seems unexceptional. In the example of Western Europe, storminess was very severe four centuries ago.

The solution to weather catastrophes–a fear of which is hard-wired in humans–is not to implement ineffective policies out of a vague concept of precaution, but to strive for scientific facts.

Dr. Sallie Baliunas served as deputy director of Mount Wilson Observatory and senior scientist at the George C. Marshall Institute in Washington, DC and chairs the Institute’s Science Advisory Board.