Global warming: in olden days a natural evolutionary process; today, ostensibly, the result of man-made increases in greenhouse gases such as carbon dioxide and methane. So, why are we even asking the question, is global warming real? Of course it is. But there are also natural causes of global warming.
As mentioned above, however, asking “what is the greenhouse effect” is a question that is historically based. Indeed, for most of its time, the earth was probably warmer than it is at present, with little or no polar ice: none of that was the cause of man-made messes.
Earth’s considerably warmer climate prior to the last million years comes from paleodata of various kinds. For almost all of the time from about 2 to at least 200 million years ago, the surface temperature exceeded that of today. The greatest warmth is found in a period, about 100 million years ago, when the mean global surface temperature may have been as much as 6 to 8° C above that of today. This was followed by a fairly steady cooling, sometimes with abruptly stepped transitions, towards the unique glacial oscillations of the last few million years.
During most of this long period of time, and certainly from about 50 to 150 million years ago, there is little evidence for ice sheets of continental scale, and subtropical plants and animals lived far poleward of their present limit of about 30°–the latitude of northern Florida. The dinosaur age, ending about 65 million years ago, overlaps most of this warm interval, and fossilized remains of these large reptiles have been found on the North Slope of Alaska. Later, during the warmest part of the mammalian age (55 million years ago), large trees grew in arctic Canada (78°N), in regions that today are covered by tundra. Alligators and primates, also indicative of warm climate, have been found on nearby Ellesmere Island. Fossils of warm-water mollusks have also been recovered on the Antarctic Peninsula.
However, as long as the continent of Antarctica exists at the southern pole of our planet we probably will be repeatedly pulled back into glacial ice ages. This occurs because ice caps, which cannot attain great thickness over open ocean, can and do achieve great thickness over a polar continent– like Antarctica. Antarctica used to be located near the equator, but over geologic time has moved by continental drift to its present location at the south pole. Once established, continental polar ice caps act like huge cold sinks, taking over the climate and growing bigger during periods of reduced solar output.
Natural Causes of Climate Change
Besides any contribution attributable to the man-made greenhouse gases, there are significant natural forces believed to affect climate. Although not fully understood, many clues to the basic causes of natural climate change and natural causes of global warming have been collected. These include:
(1) Tectonic activity. Landmass distribution, i.e. shifting continents (continental drift) cause changes in circulatory patterns of ocean currents. For instance, the Gulf Stream plays an important role carrying heat from the equator poleward. When this current is disturbed, dramatic climatic changes can occur over a short period of time. In addition, undersea activity, i.e. sea floor spreading associated with continental drift also causes variations in ocean current circulation.
(2) Astronomical activity. In the past 18,000 years, the earth’s temperature has risen approximately 16 degrees F and the sea level has risen 300 feet. The best clue for explaining these changes comes from a consideration of changes in the orbital characteristics of the Earth.
The basic premise of the theory is that, as the Earth travels through space, three separate cyclic movements combine to produce variations in the amount of solar energy falling on the Earth. Firstly, the Earth’s orbit goes from quite elliptical to nearly circular in a cycle with a period of ~100,000 years. Presently, we are in a period of low eccentricity (more circular) and this gives us a lesser seasonal change in solar energy (about ~7%). When the elliptical orbiting is at its peak, “seasonality” reaches ~20%. Thus changes in the length of seasons in each hemisphere occur through changes the length of time between the vernal and autumnal equinoxes.
A second cycle takes about 41,000 years to complete and involves changes in tilt of the Earth’s axis. The smaller the tilt, the less seasonal variation there is between summer and winter at middle and high latitudes. For small tilt, the winters would tend to be milder and the summers cooler. This would lead to more glaciation.
The third cycle is related to changes in the orientation of Earth’s rotational axis. This occurs over a ~23,000 year cycle. Presently, the Earth is closest to the Sun in January and farther away in July. Due to this factor, the reverse will be true in ~11,000 years. This will give the Northern Hemisphere more severe winters.
Overall, these cycles suggest movement toward a cooler climate in the northern hemisphere, with extensive glaciation. Other factors which work in conjunction with the Earth’s orbital changes include amount of dust in the atmosphere, reflectivity of the ice sheets, concentration of greenhouse gases, changing characteristics of clouds, rebounding of land having been depressed by ice.
(3) Atmospheric activity. Heat retention occurs due to gases such as carbon dioxide and methane in Earth’s atmosphere– the “greenhouse effect”. Solar reflectivity changes are related to cloud activity, volcanic dust and the extent of polar ice caps.
The Long and the Short About Timelines of Climate Change
A best guess about long term climate change (millions of years) is that it is associated with the very slow process of “plate tectonics” (movement of continental plates) and its influence on the atmospheric greenhouse effect. Over a time scale of 300 million years, the continental plates have moved greatly. The question remains as to why the temperatures dropped. Perhaps the answer lies in changes in the natural production rate of carbon dioxide – the number one the greenhouse gases. We know that CO2 is produced in volcanoes and in the mid-ocean trenches. It is lost by being slowly absorbed in the oceans. Both of these processes are very slow – about the right time scales to explain the great Ice Ages. One theory is that increased carbon dioxide is discharged by faster rate of sea-floor spreading due to increases in volcanic activity. Thus, volcanic-related rapid sea floor spreading promotes global warming by enriching the CO2 content of the atmosphere. Similarly, global cooling may result from stalled or slowed spreading.
Medium term (thousands of years) climate change occurred with semi-regular advances and retreats of glaciers during an individual ice age. For the past 800,000 years, large global climate oscillations have recurred in cycles of approximately 100,000 years. The warm periods, called interglacial periods, appeared to last approximately 15,000 to 20,000 years before regressing back to a cold ice age climate.
How Fast Can Global Warming Climate Change Occur?
Though the driving factors of climate change are generally thought to be long term — e.g. changes in ocean circulations, volcanic eruptions, solar variations, greenhouse gases build up — recent studies and more recent paleodata suggest quite another face of the climate system. Sometimes referred to as “abrupt transitions,” major shifts in some components of the Earth’s climate can be accomplished on time scales of decades or less.
Whatever the causes of global warming — man-made greenhouse gases or natural forces — we now know that, in at least the North Atlantic, the climate system can change very rapidly. This may also include rapid ocean circulation changes in the future, or global warming effects on the arctic.
How Global Warming Affects the Arctic.
There are no permanent ice sheets today on the North American continent, as was the case in the past, but global warming Arctic ice melting or the extensive Greenland ice cap could well influence ocean salinities. Increased precipitation over the North Atlantic, induced by warmer temperatures, could also reduce the saltiness of seawater, short-circuiting the ocean circulation in a manner similar to what occurred during the ice ages. Ocean currents and temperature patterns in the Pacific Ocean also play an important role in regulated our climate. Recent studies have suggested a decade-based oscillation in the Pacific Ocean that can effect climate across the United States. Positive phases enhance the occurrence of El Nino events and negative phases increase the occurence of La Nina events. These oceanic and atmospheric patterns can have a large impact on marine and terrestrial ecosystems.
One more thing: Human activity of course didn’t cause past climate change, but that’s no evidence that it doesn’t now.
Of course, there are other opinions…..