GGR 203 - INTRODUCTION TO CLIMATOLOGY

1.1  Definitions

1.1.1 Climatology: the study of the global climate system, including the processes responsible for maintaining climate at different scales, and a description of the climates of different regions and environments

1.1.2 System: a set of components that interact with each other.

“A” and “B” form part of a system if “A” influences “B” and “B” influences “A”

1.1.3 Climate system: Atmosphere

Oceans

Cryosphere     -    sea ice

-     snow cover

-    alpine glaciers

-    ice sheets (GI, AIS today)

Biosphere

Lithosphere (Earth’s crust)

See Table 1 for a matrix of interactions between all possible pairs of these components.

To learn this table, I suggest making a list of the kinds of effects seen for each component (rows), then learn where (which column) they apply.

The sun is not part of the climate system … .

Rather, it is an external forcing.

There is a heat flux from the interior of the Earth (due to the hot core) to the surface of about 0.3 W/m2. This flux is also an external forcing (it is external to, or not part of, the climate system) eventhough it is physically surrounded by the system components. This flux, although 1000 times smaller than the heat input from the Sun (as we’ll see later) nevertheless notable influences the climate system at the time scale of glacial-interglacial climate oscillations.

1.1.4 Climate: the mean state of the climate system plus the variability and other statistics. We can examine the mean and variability of

-temperature

- winds

- pressure

- rainfall, soil moisture

Traditionally, climate refers to the state of the climate system near the Earth’s surface

For agricultural and natural systems, the variability of temperature or rainfall from one year to the next or within the growing season can be just as important as the mean

Because climate is the statistical properties of the climate system, it needs to be based on a sample. By convention, climate statistics (means, variabilities) are based on a 30 year sample.

Thus, a change in temperature from one year to the next, or even from one decade to the next, is not a change in climate. Rather, it is part of the variability that defines that climate.

1.1.5 Meteorology: the study of the day-to-day variations in the state of the atmosphere (“weather”).

To predict the future weather, one starts from specific observed initial conditions and then one computes the evolution of that state to a specific time in the future.

Climatology, on the other hand, is concerned with means and variabilities averaged over a period of time (30 years). The climate depends strongly on the boundary conditions (solar energy coming in at the top of the atmosphere, the   nature of the land or ocean surface).

Predicting a change in the climate, therefore, is quite different from the problem of predicting the weather. The fact that we can’t predict the weather more than one week in advance is completely irrelevant to the problem of predicting changes in climate 100 years from now in response to, say, an increased in atmospheric CO2 concentration.  [hockey analogy]

1.2 Overview of past natural climatic change

Because we have defined climate in terms of both the mean and variability, then if either the mean or even just the variability of some climate variable (such as temperature) changes, the climate has changed.

Figure 1.1 in the figures file for Chapter 1 gives an overview of estimated change in global average temperature over the past 500 million years.

Key points – there were 4 episodes of several million years duration with periodic

glacial-interglacial oscillations during the past 500 million years, the mostrecent being during the past 2 million years roughly.

- at other times temperatures were 10-14°C warmer than today

- during the last 700,000 years there were 7 saw-tooth shaped glacial-interglacial cycles of about 100,000 years duration, with a gradual, oscillatory approach into full glacial conditions, followed by abrupt (within 10,000 years) transitions to interglacial conditions

This is especially evident in Figure 1.2 for the past 400,000 yrs, from which it can also be seen that:

- atmospheric CO2  and CH4  (methane) concentrations varied as well, in such away as to reinforce the temperature changes (lower concentrations when it was getting   colder, and viceversa).

-the last ice age ended around 10,000 yrs ago, and temperatures reached a peak (maybe 1°C warmer than during the late 1800s) about 6000 years ago

Figure 1.3 compares lake level status (low, intermediate, and high), as deduced from geomorphic and other evidence, for two time periods compared to present: the peak of the last ice age (about 18,000 years ago) and the mid Holocene (6000 years ago). US SW much moister than now during the last ice age (with huge lakes where there are now only small remnants), while the Sahara desert and east Africa were much moister just 6000 yrs ago.

Returning to temperature, as seen from Figures 1.4 to 1.6,

- there had been a downward trend of about 0.2°C over the period AD 1000-1900

- the climate warmed by almost 1.0 C during the past 100 years (this is due without  question to human emissions of CO2  and other greenhouse gases to the atmosphere)

- the warming trend has been particularly large in polar regions

1.3       Overview of the present climate

Take note of the following information from the indicated figures:

Fig 1.7 – all layers and boundaries; temperature, heights and pressures of 1st  3 boundaries

Fig 1.8 – 1-cell early view vs 3 cells (names, locations of the 3 cells and the direction of   flow), names and directions of winds, names and locations of high and low pressure cells; qualitative variation of zonal mean surface P with latitude

Fig 1.9 – trade winds location, ITCZ as convergence of trade winds, shifts in location with seasons

Fig 1.10 – monsoon regions, locations with winter rain, summer rain, and double rain Fig 1.11 –seasonal reversal of winds over Tibetan plateau and east Asia

Fig 1.12 – the westerly jet stream – note shift in position and strength with seasons (poleward and stronger in winter, greater variation in NH than in SH)

Fig 1.13 – cross-section shows E-W average of the E-W (zonal) wind, where positive is from the west. Seasonal variation in position and strength are seen (much stronger and further equatorward during winter, especially in the NH) (much less variation in the SH). Notice easterly winds (negative values) in stratosphere in summer in both hemispheres

Fig 1.14 – January and July temperature patterns: large changes in polar regions, small changes in tropical regions, so there is a large equator-to-pole temp difference in winter, small difference in summer

Fig 1.15 – surface pressure pattern in Jan and July:  huge and strong high-pressure cell over Siberia in January, turns into weak low-pressure cell in July. Large lows in January centred over Iceland and the Aleutian Islands, largely gone in July. Strong high-pressure cell over N Atlantic in July (Azores High). High-pressure cells over mid latitude oceans, strongest in summer in both hemisphere.

Fig 1.16 – seasonal rainfall: ITCZ and march of the monsoons is evident

Fig 1.17 – rainfall extremes. Relative to the average precipitation, the extremes are strongest in dry regions. i.e., in the desert, it either doesn’train, or it pours.

Fig 1.18 – an example of the alternating pattern of extreme warm and extreme cold regions due to a distortion in the airflow. When there is strong airflow from the north somewhere (bringing cold weather), there has to be compensating strong airflow at some other longitude (bringing warm weather)

1.4       Physical basis of climate

We can subdivide the processes responsible for determining the state of the climate system into:

Radiative processes, involving:         - solar radiation

- Infrared radiation

Dynamics: -atmospheric and oceanic motions (winds and currents)

- Flow of ice sheets, crustal motions

Thermodynamics – deals with heat, internal energy, and work

- leads to the study of the vertical stability of the atmosphere

- leads to important relationships involving evaporation and absorbed energy at the Earth’s surface

Surface processes – evaporation

- exchange of heat and momentum with the atmosphere

- occurrence of ice and snow

Clouds are extremely important, as they strongly affect, and are affected by, all of the above sets of processes.

At yearly and longer time scales, biological processes play a very important role in climate

At geological time scales, coupled biogeochemical cycles also play a very important role. For example, the coupled carbon-phosphate cycles.