Introduction to Climate
This section provides a brief overview of the earth's climate. The general concepts found in this section include the following:
- The general distinctions between weather and climate.
- Daily weather measurements are highly variable compared to long-term climate data making it difficult to detect long-term trends based on limited data.
- Long-term climate averages are the result of significant annual climate variability. Random climate variability makes detecting climate change more difficult.
- The earth's climate has changed over time and will continue to change. Paleoclimatologists study the earth's climate over millions of years using a variety of methods. Data for past climate changes can be gathered from sources beyond long-term weather observations.
- Plant pollen is physically distinct in size and shape in different plant species and may survive for long periods of time in sediment deposits, depositing a record of plant species in the area. The study of plant pollen trapped in lake sediments and rocks is called palynology.
- Dendrochronology is the study of past climate through tree rings and allows scientists to observe the direct impact of climate on annual tree growth patterns.
- The study of ice cores allows us to study ancient climates by chemical and physical analyses.
- Remote sensing is a valuable tool for analyzing the current state of the biosphere.
The earth's climate is generally defined as the average weather over a long period of time. A place or region's climate is determined by both natural and anthropogenic (human-made) factors. The natural elements include the atmosphere, geosphere, hydrosphere, and biosphere, while the human factors can include land and resource uses. Changes in any of these factors can cause local, regional, or even global changes in the climate.
Systematic view of the components of the earth’s global weather and climate system which involves mutual interactions between components of the atmosphere, hydrosphere, lithosphere and biosphere. Figure from IPCC WGI AR4 Chapter 1.
Weather is defined as the state of the atmosphere at a given time and place, with respect to variables such as temperature, moisture, wind speed and direction, and barometric pressure.
Climate is defined by NOAA as the expected frequency of specific states of the atmosphere, ocean, and land including variables such as temperature (land, ocean, and atmosphere), salinity (oceans), soil moisture (land), wind speed and direction (atmosphere), current strength and direction (oceans), etc. Climate encompasses the weather over different periods of time and also relates to mutual interactions between the components of the earth system (e.g., atmospheric composition, volcanic eruptions, changes in the earth’s orbit around the sun, changes in the energy from the sun itself, etc.).
To illustrate the difference between weather and climate, consider forecasts: Weather forecasts are event, location and day or even minute specific. Climate forecasts take advantage of the slow evolution of the coupled atmosphere/ocean/land/cryosphere system, and are usually expressed in probabilistic terms (e.g. probability of warmer or wetter than average conditions) for periods such as weeks, months or seasons. Note that climate forecasts never predict specific weather events. Or consider Normals: Climate Normals provide estimates of the maximum and minimum temperatures typical of a given location primarily based on analysis of historical data while the actual temperatures observed on that day in any particular year are considered weather.
While the climate definition emphasizes the mutual interactions between components of the earth system, it is important to note that they also occur in weather.
Interest in accurate and timely weather and climate observations has grown tremendously over the past decade. Applications range from managing multi-billion dollar economic weather risk for business and industry to understanding the impact of climate variability. Assuring data quality from the point of observation to the point of delivery is critical. The National Oceanic and Atmospheric Administration (NOAA) plays a critical role in the collection, quality control, archiving, and dissemination of accurate, secure surface climate and weather observations. These data are used by the agency to fulfill its mandate to describe the nation’s climate and detect, monitor and predict climate variability and change, including characterization of socio-economic impacts. Customers consider NOAA the neutral broker” for climate data services.
Paleoclimates (Ancient Climates)
Good weather records extend back only about 130 years. In that time, the earth's global average temperature has increased by approximately 0.5 degrees centigrade or about 1 degree Farenheit.
Scientists have studied the earth's past climate extending back millions of years. To examine these long time scales, known as geologic time, scientists have had to gather clues from geologic and plant fossil records.
Evidence in the fossil record indicates that the earth's climate in the distant past was very different from today's. However, the climate has fluctuated substantially within the last several centuries - too recent to be reflected in the fossil record. Studying past climates and climate changes help us to better understand our current climate and what may happen in the future.
The earth's atmosphere has evolved over the course of its long history (approximately 4.5 billion years), resulting in significant changes to global, regional, and local climates. Some of the fossil record suggests that these changes are somewhat cyclical, with periods of global warming followed by ice ages. The most recent global ice age ended about 18,000 years ago with a gradual warming since then despite some intermediate periods of cooling.
Paleoclimates and Pollen
Modified with permission from Global Climates - Past, Present, and Future, S. Henderson, S. Holman, and L. Mortensen (Eds.). EPA Report No. EPA/600/R-93/126, U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC. 25 - 38.
Evidence found in the fossil record indicates that in the distant past, the earth's climate was very different than it is today. There have also been substantial climatic fluctuations within the last several centuries, too recently for the changes to be reflected in the fossil record. Since these changes are important to understanding potential future climate change, scientists have developed methods to study the climate of the recent past.
Although human-recorded weather records cover only the last few hundred years or so, paleoclimatologists and paleobotanists have found ways of identifying the kinds of plants that grew in a given area, from which they can infer the kind of climate that must have prevailed. Because plants are generally distributed across the landscape based on temperature and precipitation patterns, plant communities change as these climatic factors change. By knowing the conditions that plants preferred, scientists can make general conclusions about the past climate.
How do paleobotanists map plant distribution over time? One way is to study the pollen left in lake sediments by wind-pollinated plants that once grew in the lake's vicinity. Sediment in the bottom of lakes is ideal for determining pollen changes over time because it tends to be laid down in annual layers (much like trees grow annual rings). Each layer traps the pollen that sank into the lake or was carried into it by stream flow that year.
To look at the "pollen history" of a lake, scientists collect long cores of lake sediment, using tubes approximately 5 centimeters (cm) in diameter. The cores can be 10 m long or longer, depending upon the age of the lake and amount of sediment that's been deposited. The removed core is sampled every 10 to 20 cm and washed in solutions of very strong, corrosive chemicals, such as potassium hydroxide, hydrochloric acid, and hydrogen fluoride. This harsh process removes the organic and mineral particles in the sample except for the pollen, which is composed of some of the most chemically resistant organic compounds in nature. Microscope slides are made of the remaining pollen and examined to count and identify the pollen grains.
Because every plant species has a distinctive pollen shape, botanists can identify from which plant the pollen came. Through pollen analysis, botanists can estimate the composition of a lake area by comparing the relative amount of pollen each species contributes to the whole pollen sample. Carbon dating of the lake sediment cores gives an approximate age of the sample.
Scientists can infer the climate of the layer being studied by relating it to the current climatic preferences of the same plants. For example, they can infer that a sediment layer with large amounts of western red cedar pollen was deposited during a cool, wet climatic period, because those are the current conditions most conducive to the growth of that species.
Why are scientists who study climate change interested in past climates? First, by examining the pattern of plant changes over time, they can determine how long it took for plant species to migrate into or out of a given area due to natural processes of climate change. This information makes it easier to predict the speed with which plant communities might change in response to future climate change. Second, by determining the kinds of plants that existed in an area when the climate was warmer than at present, scientists can more accurately predict which plants will be most likely to thrive if the climate warms again.
Time & Cycles - Dendrochronology
Modified with permission from Global Change: Time and Cycles, Department of the Interior, U.S. Geological Survey, Reston, VA, USA.
Trees contain some of nature's most accurate evidence of the past. Their growth layers, appearing as rings in the cross section of the tree trunk, record evidence of floods, droughts, insect attacks, lightning strikes, and even earthquakes.
Each year, a tree adds to its girth, the new growth being called a tree ring. Tree growth depends upon local conditions such as water availability. Because the amount of water available to the tree varies from year to year, scientists can use tree-ring patterns to reconstruct regional patterns of drought and climatic change. This field of study, known as dendrochronology, was begun in the early 1900s by an American astronomer named Andrew Ellicott Douglass.
A tree ring consists of two layers:
A light colored layer grows in the spring
A dark colored layer in late summer
During wet, cool years, most trees grow more than during hot, dry years and the rings are wider. Drought or a severe winter can cause narrower rings. If the rings are a consistent width throughout the tree, the climate was the same year after year. By counting the rings of a tree, we can pretty accurately determine the age and health of the tree and the growing season of each year.
Modern dendrochronologists seldom cut down a tree to analyze its rings. Instead, core samples are extracted using a borer that's screwed into the tree and pulled out, bringing with it a straw-size sample of wood about 4 millimeters in diameter. The hole in the tree is then sealed to prevent disease.
Computer analysis and other methods have allowed scientists to better understand certain large-scale climatic changes that have occurred in past centuries. These methods also make highly localized analyses possible. For example, archaeologists use tree rings to date timber from log cabins and Native American pueblos by matching the rings from the cut timbers of homes to rings in very old trees nearby. Matching these patterns can show the year a tree was cut, thus revealing the age of a dwelling.
Dendroclimatology is actually the science that uses tree rings to study present climate and reconstruct past climate. Example: analyzing ring widths of trees to determine how much rainfall fell per year long before weather records were kept.
Ice cores drilled from deep within ice sheets or glaciers give us our longest look back in time—as far back as 800,000 years, so far. Analyses of the water molecules, air bubbles, and material such as ash and dust can provide information on local temperatures, greenhouse gases, volcanic eruptions, and other factors that influence climate.
Many materials can appear in an ice core. Layers can be measured in several ways to identify changes in composition. Small meteorites may be embedded in the ice. Volcanic eruptions leave identifiable ash layers. Dust in the core can be linked to increased desert area or wind speed.
Isotopic analysis of the ice in the core can be linked to temperature and global sea level variations. Analysis of the air contained in bubbles in the ice can reveal the palaeocomposition of the atmosphere, in particular, $CO_2$ variations.
Remote Sensing and the Aquarius Project to Measure SSS
Three decades of scientific and technical development have made it possible to accurately measure Sea Surface Salinity (SSS) from 657 kilometers (408 miles) above Earth's surface. Aquarius' measurements of SSS will provide a new perspective on the ocean and its links to climate, greatly expanding upon extremely limited past measurements. Moreover, the accuracy of the data -- equivalent to a "pinch" of salt in a gallon of water -- will help to answer several outstanding scientific questions:
- Why is the Atlantic the saltiest ocean?
- How do changes in rainfall over the tropical oceans -- and the resulting effects on SSS -- influence the development of monsoons?
- How can we use SSS data to improve climate models? El Niño and La Niña forecasts?
- What is the variability of thermohaline (i.e., temperature and salinity-controlled) circulation, often depicted as a "Global Conveyor Belt"?
- How might changes in the "Global Conveyor Belt" lead to climate change in Europe and other areas?
Global Conveyor Belt
Aquarius is a mission of original exploration:
- Salinity has been sparsely detected at sea, limited mostly to summertime observations in shipping lanes.
- Aquarius will collect as many sea surface salinity measurements as the entire 125-year historical record from ships and buoys
- Aquarius will be able to resolve global SSS changes from month-to-month, season-to-season, and year-to-year
The data-related goals of Aquarius are:
- Provide the first global observations of SSS, covering Earth's surface once every 7 days
- Deliver monthly 150-kilometer resolution SSS maps over a 3-year mission lifetime
- Achieve SSS accuracy of 0.2 psu: this is about a "pinch" (i.e., 1/8th of a teaspoon) of salt in 1 gallon of water
The research-related goals of Aquarius include a better understanding of:
The water cycle - 86% of global evaporation and 78% of global precipitation occur over the ocean; thus SSS is the key variable for understanding how fresh water input and output affects ocean dynamics
Ocean circulation - With temperature, salinity determines seawater density and buoyancy, driving the extent of ocean stratification, mixing, and water mass formation
Climate - As computer models evolve, Aquarius will provide the essential SSS data needed to link the two major components of the climate system: the water cycle and ocean circulation
Here's a short video from NASA concerning the Aquarius Project:
Many other projects are underway for the discovery of present and in some cases, can provide data as to former climates as well.
To investigate the extent, speed, and effects of historical climate changes locally and globally, scientists rely on data collected from tree rings, ice cores, pollen samples, and the fossil record. Remote sensing and ground observations provide current data and computers are used to detect possible patterns and cycles from these sources. large databases allow scientists to reconstruct maps of former regional climates, and reveal when, where, and how quickly climates may change.
Further Reading and Resources
Ice core data from NOAA: http://www.ncdc.noaa.gov/paleo/icecore/trop/quelccaya/quelccaya.html
atmosphere : Gaseous layer surrounding a planet; the whole mass of air surrounding the earth.
climate : The prevailing or normal pattern of weather at a place, or in a region, averaged over a long period of time; in contrast to weather, which is the state of the atmosphere at a particular time.
conductivity : A measure of the ability of a material to conduct or transmit an electric charge.
"Global Conveyor Belt": A simplified illustration of the integrated system of surface and deep-ocean currents that move waters from the polar regions, throughout the ocean, and return them to polar regions where the deep-ocean water masses form.
model : System of data, inferences, and relationships, presented as a description of a process or entity.
monsoon : A name for seasonal winds derived from the Arabic word for season, mausim. The term was originally applied to winds over the Arabian Sea that blow from the southwest during summer and the northeast during winter, subsequently extended to similar seasonal winds in other parts of the world.
practical salinity unit (psu): Used to describe the concentration of dissolved salts in water, the UNESCO Practical Salinity Scale of 1978 (PSS78) defines salinity in terms of a conductivity ratio, so it is dimensionless. Salinity was formerly expressed in terms of parts per thousand (ppt) or by weight (parts per thousand or 0/00). That is, a salinity of 35 ppt meant 35 pounds of salt per 1,000 pounds of seawater. Open ocean salinities are generally in the range between 32 and 37.
salinity : A measure of the quantity of dissolved solids in ocean water. In general, salinity reflects the total amount of dissolved solids in ocean water in parts per thousand by weight after all carbonate has been converted to oxide, the bromide and iodide to chloride, and all the organic matter oxidized. Salinity is now measured as pratical salinity units (psu).
system : 1) A regularly interacting or interdependent group of items forming a unified whole. 2) A manner of classifying. 3) A group of interacting bodies under the influence of related forces.
thermohaline circulation : The vertical movement of ocean water driven by density differences resulting from the combined effects of variations in temperature and salinity.
variability : The quality of being subject to change or deviation from a norm or standard.
water mass: A body of water identifiable by its temperature, salinity, or chemical content.