CHAPTER FIVE - THE CLIMATE OF ETHIOPIA AND THE HORN
Chapter's Objectives
Upon the completion of this chapter, you will be able to:
Distinguish between weather and climate,
Explicate the spatiotemporal patterns and distribution of temperature and rainfall in Ethiopia,
Analyse climate and its implications on biophysical and socioeconomicaspects,
Comprehend the causes, consequences andresponse mechanisms of climate change.
5.1 Introduction
Meaning of weather and Climate
Weather is the condition of atmosphere over a short period of time. In general, the weather that impacts the surface of the Earth and those that live on the surface takes place in the troposphere.
Climate is the state of the atmosphere over long time periods, decades and more. It is the composite of daily weather conditions recorded for long periods of time. Climate also takes into account the extremes or variations that may occur beyond the average conditions.
Both weather and climate are composed of the following elements: temperature, air pressure, humidity, wind, sunshine, precipitation etc.
Ethiopia as a large country in the Horn of Africa is characterized by a wide variety of altitudinal ranges and diverse climatic conditions. In addition, because of its closeness to the equator and the Indian Ocean, the country is subjected to large temporal and spatial variations in elements of weather and climate.The climate of Ethiopia is therefore mainly controlled by the seasonal migration of the Intertropical Convergence Zone (ITCZ) and associated atmospheric circulations as well as by the complex topography of the country.
5.2. Controls of Weather and Climate
The distribution of weather and climate elements over the surface of the earth is uneven in terms of magnitude and time. Hotness or coldness, rainy or cloudiness, sunniness, windiness or calmness, of air you are feeling on the daily base in your current location are expressions of weather. Now the question one should inquire here is what determines the variations in weather and climate between places and seasons. Hence, these determining factors are called controls of weather and climate or climatic controls. The weather and climate of any particular location on earth is determined by a combination of many interacting factors. These include latitude, altitude/elevation, revolution of the earth and inclination of the earth’s axis, nearby water/ distance from water bodies, ocean currents, topography/ mountain barriers, vegetation, and prevailing winds. Moreover, the global climate system and any changes that occur within it also influence local climate.
Latitude is the distance of a location from the equator. The sun shines directly on equator for more hours during the year than anywhere else. As you move further away from the equator towards the poles, less solar insolation is received during the year and the temperature become colder. When we speak of the latitudinal impact on the climate of Ethiopia and the Horn, we are considering the intensity of temperature in the region.
Ethiopia and the Horn’s location within the tropical zone resulted in;
- High average temperatures during most of the year,
- High daily and small annual ranges of temperature,
- No significant variation in length of day and night between summer and winter.
The earth’s rotation axis makes an angle of about 66 ½° with the plane of its orbit around the sun, or about 23½° from the perpendicular to the ecliptic plane. This inclination determines the location of the Tropics of Cancer, Capricorn and the Arctic and Antarctic Circles. As the earth revolves around the sun, this inclination produces a change in the directness of the sun’s rays; which in turn causes the directness of the sun and differences in length of day cause seasons. These different seasons result in the temporal variation of temperature in a year in Ethiopia and in the Horn as a whole.
Equinoxes and Solstices
An equinox is the instant of time when the sun strikes the plane of the Earth’s equator. During this passage the length of day and night are equal. Moreover, revolution of the earth along its orbit, the inclination of its axis from the plane of that orbit, and the constant position(parallelism) of the axis causes seasonal changes in the daylight and darkness periods. Equinox appears twice a year. Let’s see two major equinoxes’;
The Vernal (spring) equinox: is the day when the point of verticality of sun’s rays crosses the equator northwards. This equinox experiences in Northern Hemisphere when the sun is exactly above the equator. During this period, the length of day and night are equal. Vernal (spring) equinox marks the beginning of Spring season. March 21 marks the offset of the vernal equinox. The Autumn equinox: appears to happen when the sun crosses equator giving approximately equal length between day and night. It appears to happen when the visible sun moves south across the celestial equator on 23rd of September. It marks the beginning of Autumn season.
Solstice is an event when the overhead sun appears to cross northern or southern points relative to the celestial equator resulting in unequal length of days and nights in the hemispheres. Both hemispheres during this event has either the most or least sunlight of the year.
The summer Solstice: on June 21st, the northern hemisphere has maximum tilt towards the sun experiencing longest daylight of the year. It is the astronomical first day of summer in the Northern Hemisphere. The sun is at its highest position in the noonday sky, directly above 23 ½ in the Tropic of Cancer.
The winter solstice: 22ndof December is the day when the maximum southward inclination is attained in the Southern Hemisphere. In this event the sun travels shortest length causing longest night and shortest daylight. In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23 ½ ° south of the equator.
Altitude is the height of location above the mean sea level. Under normal conditions there is a general decrease in temperature with increasing elevation. The average rate at which temperature changes per unit of altitudinal change is known as lapse rate. The lapse rate is limited to thelower layer of the atmosphere named as troposphere. The normal lapse rate is 6.5°C per kilometer rise in altitude.
Altitude is the main factor that determines the spatial distribution of temperature in Ethiopia. Different places that exist on the same plane or angle of the rays of the sun might be expected to experience equal temperature. However, due to the impact of altitude, they do not.
For example, three Ethiopian cities, Bako, Addis Ababa, and Awash all lies on the 90 N latitude, and therefore they might be expected to receive equal magnitudes of direct rays from the sun and therefor equal temperature. However, their altitudes very, and therefore their temperatures vary, as shown in table 5.1.
5.1 the role of altitude in modifying tempreture
City | Altitude | Average annual temperature |
Bako (W. Shoa) | 1800 | 17oc |
Addis Ababa | 2200 | 16oc |
Awash | 900 | >25oc |
5.3. Spatiotemporal Patterns and Distribution of Temperature and Rainfall in Ethiopia
Altitude is an important element in determining temperature of Ethiopia and the Horn. Latitude, humidity and winds, with varying magnitude have also significant impacts on temperature conditions in Ethiopia.
The spatial distribution of temperature in Ethiopia is primarily determined by altitude and latitude. The location of Ethiopia at close proximity to equator, a zone of maximum insolation, resulted for every part of the country to experience overhead sun twice a year. However, inEthiopia, as it is a highland country, tropical temperature conditions have no full spatial coverage. They are limited to the lowlands in the peripheries.
Away from the peripheries the land begins to rise gradually and considerably, culminating in peaks in various parts of the country. Thus temperature, as it is affected by altitude, decreases towards the interior highlands. Mean annual temperature varies from over 30Cin the tropical lowlands to less than 100c at very high altitudes.
The Bale Mountains are among highlands where lowest mean annual temperatures are recorded. The highest mean maximum temperature in the country is recorded in the Afar Depression.Moreover, lowlands of north-western, western and south-eastern Ethiopian experiences mean maximum temperatures of more than 300C.Environmental influences have their own traditional expressions in Ethiopia and there are local terms denoting temperature zones as shown in the table below:
Table 5.2: Temperature versus Altitude
|
Altitude in Meters |
Mean annual temperature in 0c |
Traditional agroecological name |
Global equivalence |
|
3,300 and above |
Below 10 |
Wurch |
Alpine or Afro Alpine/cool |
|
2,300-3,300 |
>11.5-17.5 |
Dega |
Temperate/Cool Temperate |
|
1,500-2,300 |
>17.5-20 |
WoinaDega |
Subtropical/Temperate |
|
500-1,500 |
>20.0-27.5 |
Kola |
Tropical/Warm Temperate |
|
Below 500 |
> 27.5 |
Berha |
Desert/ Hot |
Source: NRMRD MoA, 2000
The temporal distribution of Ethiopian temperature is characterized by extremes.The major controls determining its distributions are latitude and cloud cover. However, some parts of the country enjoy a temperate climate. In the tropics, the daily range of temperature is higher and the annual range is small, whereas the reverse is true in the temperate latitudes. In Ethiopia, as in all places in the tropics, the air is frost free and changes in solar angles are small making intense solar radiation.
Ethiopia’s daily temperatures are more extreme than its annual averages. Daily maximum temperature varies from a high of more than 370c over the lowlands in northeast and southeast to a low of about 10 -150c over the northwestern and southwestern highlands. The variation in the amount of solar radiation received daily is small throughout the year. As already explained, temperature is high during the daytime in some places, and is considerably reduced at night resulting maximum difference in the daily range.
But in the case of monthly averages, variation is minimal and the annual range of temperature is small. This holds true in both the highlands and lowlands. In Ethiopia and elsewhere in the Horn, temperature shows seasonal variations. For example, months from March to June in Ethiopia haverecords of highest temperatures. Conversely, low temperatures are recorded from November to February.
It is not easy to observe distinct variation in temperature between seasons as the sun is always high in the tropics. However, there is a slight temperature increase in summer. Southern part of Ethiopia receives highest records of temperature in autumn and spring following the relative shift of the sun; whereas in the northern part of the country, summer season is characterized by higher temperature.
It has to be noted that certain seasons should have special considerations. For instance, unlike other parts of Ethiopia, the southern and southwestern highlands experience reduced temperature. This is because the temperature and the amount of energy reaching the surface is directly related with the directness of the sun.The direction of rain bearing winds (leeward or windward side) also determines the temperature variations in mountainous regions.
Rainfall system in Ethiopia is characterized by complexities. To encompass the system, it needs an understanding of the position of Inter Tropical Convergence Zone (ITC), pressure cells, and Trade Winds.Rainfall in Ethiopia is the result is influenced by the position of Intertropical Convergence Zone (ITCZ). The convergence of Northeast Trade winds and the Equatorial Westerlies forms the ITCZ, which is a low-pressure zone. The inter-annual oscillation of the surface position of theITCZ causes a variation in the Wind flow patterns over Ethiopia and the Horn. Following the position of the overhead sun, the ITCZ shifts north and south of the equator.As the shift takes place, equatorial westerlies from the south and southwest invade most parts of Ethiopia bringing moist winds.However, these winds decrease the length of rainy seasons and magnitudes on the line of the shift. The shift takes place when the trade winds from the north retreat giving the space forequatorial westerlies. This development mainly happens in July in Ethiopia and the Horn causing variability and seasonality.
The ITCZ shifts towards south of equator (Tropic of Capricorn) in January. During this period, the Northeast Trade Winds carrying non-moisture-laden dominates the region. Afar and parts of Eritrean coastal areas experience rainfall in this period. Following the directness of the Sun in March and September around the equatorthe ITCZ shifts towards equator. During this time, the central highlands, southeastern highlands and lowlands receives rainfall as the south easterlies bring moist winds.
Seasonal or Temporal Variability
The rainfall is highly variable both in amount and distribution across regions and seasons. The seasonal and annual rainfall variations are results of the macro-scale pressure systems and monsoon flows which are related to the changes in the pressure systems. The temporal variability of rainfall is characterized by;
- Summer (June, July, August):From mid-June to mid-September, majority of Ethiopian regions, except lowlands in Afar and Southeast, receive rainfall during the summer season as the sun overheads north of the equator. High pressure cells develop on the Atlantic and Indian Oceans around the tropic of Capricorn Although, the Atlantic contributes a lot, the Indian Oceans is also sources of rainfall. During this season, Ethiopia and the Horn come under the influence of the Equatorial Westerlies (Guineamonsoon) and Easterlies. Hence, the Guinea monsoon and the South easterly winds are responsible for the rain in this season.
- Autumn (September, October and November):Autumn is the season of the year between summer and winter. The exact position of the ITCZ changes over the course of the year, oscillating across the equator. In autumn the ITCZ shifts towards the equator weakening the equatorial westerlies. During this season, the south easterlies from Indian ocean showers the lowlands in southeastern part of Ethiopia.
- Winter (December, January and February): In winter, the overhead sun is far south of equator. During this season, Northeasterly winds originating from the landmass of Asia dominantly prevail Ethiopian landmass. However, it has no significant coverage compared to other seasons. The northeasterly winds crossing the Red Sea carry very little moisture and supplies rain only to the Afar lowlands and the Red Sea coastal areas.
- Spring (March, April and May):In this season, the noonday sun is shining directly on the equator while shifting north from south. The shift of the ITCZ, results in longer days and more direct solar radiation providing warmer weather for the northern world. In this season, the effect of the northeast trade wind is very much reduced. Conversely, the southeaster lies from the Indian Ocean provide rain to the highlands of Somalia, and to the central and southeastern lowlands and highlands of Ethiopia.
Rainfall Regions of Ethiopia
Based on rainfall distribution, both in space and time, four rainfall regions can be identified in Ethiopia and the Horn. These are:
- Summer rainfall region:This region comprises almost all parts of the country, except the southeastern and northeastern lowlands. The region experiences most of its rain during summer (kiremt), while some places also receive spring (Belg) rain. The region is divided in to dry and wet summer rainfall regions. Hence, the wet corresponds to the area having rainfall of 1,000 mm or more. The High altitudes and the windward side experience such rainfall amount.
- All year-round rainfall region:It has many rainy days than any part of the country. It is a rainfall region in the southwestern part of the country comprising the highlands of Wellega, KeffaIlubabor and GamoGoffa. The wetness of this region is particularly due to the prepotency of moist air currents of equatorial Westerlies called theGuinea Monsoons. Both duration and amount of rainfall decreases as we move from southwest to north and eastwards. Months in summer gain highest rainfall whereas the winter months receive the reduced amount. The average rainfall in the region varies from 1,400 to over 2,200 mm/year.
- Autumn and Spring rainfall regions: The region comprises areas receiving rain following the influence of southeasterly winds. South eastern lowlands of Ethiopia up to the Somalia coasts receive rain during autumn and spring seasons when both the north easterlies and equatorial westerlies are weak. The south-easterlies bring rainfall from the Indian Ocean. About 60 percent of the rain is in autumn and 40 percent in spring. The average rainfall varies from less than 500 to 1,000 mm. Example: Gode, Moyalle, Jijiga, Yabello.
- Winter rainfall region: This rainfall region receives rain from the northeasterly winds. During the winter season, the Red sea escarpments and some parts of the Afar regionand Eritrea receive their main rain. Example: Mitswa, Assaita and Djibouti
5.4 Agro-ecological Zones of Ethiopia
As a result of the diversified altitude and climatic conditions, Ethiopia possesses diversagroclimatic zones. These zones have traditionally been defined in terms of temperature. This system divides the nation into five major climatic zones namely Bereha, Kolla, WoinaDega, Degaand Wurch. A description on each of the zones is presented as follows.
Table: Agroecological Zones of Ethiopia
|
Altitude in Meters |
Mean annual temperature in 0c |
Traditional agroecological name |
Global equivalence |
Length of growing periods (days) |
Area share (%) |
|
3,300 and above |
Below 10 |
Wurch |
Alpine or Afro Alpine/cool |
211–365 |
0.98 |
|
2,300-3,300 |
>11.5-17.5 |
Dega |
Temperate/Cool Temperate |
121–210 |
9.94 |
|
1,500-2,300 |
>17.5-20 |
WoinaDega |
Subtropical/Temperate |
91–120 |
26.75 |
|
500-1,500 |
>20.0-27.5 |
Kola |
Tropical/Warm Temperate |
46–90 |
52.94 |
|
Below 500 |
> 27.5 |
Berha |
Desert/ Hot |
0–45 |
9.34 |
Source: NRMRD MoA, 1998
The Wurch Zone
The Wurch-zone is an area having altitude higher than 3,200 meters above sea level and mean annual temperature of less than 100C. Mountains having typically fitting characteristics of this zone include mountain systems of RasDashen in SemineGonder, Guna in South Gonder, Megezez in North Shoa, Batu in Bale, Choke, AbuneYoseph, etc.
Dega Zone
This is a zone of highlands having relatively higher temperature and lower altitude compared to the wurch Zones. In Ethiopia, the Dega-zone is long inhabited and has dense human settlement due to reliable rainfall for agriculture and absence of vector-borne diseases such as malaria.Due to this high concentration of human population, the Dega zone has been intensively cultivated and has a high rate of soil erosion, overgrazing and deforestation.
WeynaDega Zone
This zone has warmer temperature and moderate rainfall. It lies between 1500-2,300 meters above sea level. It is the second largest zone covering more than 26% of the landmass of Ethiopia. The temperature and rainfall of this category is highly suitable for majority of crops grown in Ethiopia. Hence, the zone includes most of the agricultural land. The WeynaDega zone has also two growing seasons.
Kolla Zone
In Ethiopia, the geographic peripheries in south, southeast, west and northeastern part are mainly in this category. Kolla is the climate of the hot lowlands with an altitudinal range of 500 to 1500 meters above sea level. Average annual temperature ranges between 20oc and 30oc. Although mean annual rainfall is erratic, it can be as high as 1500 mm in the wet western lowlands of Gambella. Rainfall is highly variable from year to year. The region is boundary between the hot arid (Bereha) and the humid climates (WoinaDega).
Bereha Zone
Bereha is the hot arid climate of the desert lowlands. The Bereha agro-climatic zone is largely confined to lowland areas with altitude of lower than 500 meters. Around Danakil depression, the elevation goes below the sea level. Its average annual rainfall is less than 200 mm, and average annual temperature is over 27.5oc. Strong wind, high temperature, low relative humidity, and little cloud cover usually characterize Bereha. Evapotranspiration is always in excess of rainfall. Djibouti, majority of Somalia, and coastal areas of Eritrea are categorized under Kolla and Bereha zones.
5.5. Climate Change/Global Warming: Causes, Consequences and Response Mechanisms
Climate change refers to a change in the state of the climate that can be identified (e.g. using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer. It refers to any change in climate over time, due to either natural variability or human activities.
Besides spatial and temporal variations in different parts of the country, Ethiopian climate experiences extremes such as drought, flood etc. Ethiopia ranked 5thout of 184 countries in terms of its risk of drought. In the country, 12 extreme drought events were recorded between 1900 and 2010. Among the 12, seven of the drought events occurred since 1980. The majority of these resulted in famines. The severe drought of 2015-2016 was exacerbated by the strongest El Nino that caused successive harvest failures and widespread livestock deaths in some regions.
Trends in Temperature Variability
Over the last decades, Ethiopia has experienced climatic changes. Mean annual temperature has shown 0.2°C to 0.28°C rise per decade over the last 40-50 years. A rise in average temperature of about 1.3°C has been observed between 1960 and 2006. The rise has spatial and temporalvariation. Higher rise in temperature was noted in drier areas in northeast and southeast part of the country. Notably the variability is higher in July-September. The number of ‘hot days’ and ‘hot nights’ has also shown increment. Consequently, the country’s minimum temperature has increased with 0.37°C to 0.4°C per decade.
Trends in Rainfall Variability
Precipitation has remained fairly stable over the last 50 years when averaged over the country. However, these averages do not reflect local conditions which are extremely divergent and the natural variability in rainfall in the country makes it difficult to detect long-term trends.Rainfall variability is increasing (and predictability is decreasing) in many parts of the country. In some regions, total average rainfall is showing decline. For instance, parts of southern, southwestern and south-eastern regions receiving spring and summer rainfall have shown decline by 15-20%between 1975 and 2010. This has strong implications for crop production, which becomes clear when assessing the change in areas that receive sufficient rain to support cropproduction.
Changes in temperature and rainfall increase the frequency and severity of extreme events. Major floods have been a common occurrence, leading to loss of life and property in numerous parts of the country. Warming has exacerbated droughts, and desertification in the lowlands of the country is expanding.
The causes of climate change are generally categorized as anthropogenic/manmade and natural causes.
- Natural Causes
Climate change has many natural causes, such as variations in the energy budget, the position of Earth relative to Sun, the position of continents relative to the equator, and even whether the continents are together or apart. Here are some of the major natural causes:
- Earth orbital changes: The earth is tilted at an angle of 23.5° to the perpendicular plane of its orbital path. Changes in the tilt of the earth can lead to small but climatically important changes in the strength of the seasons. More tilt means warmer summers and colder winters.
- Energy Budget: Although the Sun’s energy output appears constant, small changes over an extended period of time can lead to climate changes. Since the Sun was born, 4.55 billion years ago, the star has been very gradually increasing its amount of radiation so that it is now 20% to 30% more intense than it was once.
- Volcanic eruptions: volcanic eruption releases large volumes of Sulfur dioxide, carbon dioxide, water vapor, dust, and ash into the atmosphere. The release of large volume of gases and ash can increase planetary reflectivity causing atmospheric cooling.
- Anthropogenic Causes
The growing influence of human activities on the environment is being increasingly recognized, and concern over the potential for global warming caused by such anthropogenic effects is growing. The warming of earth planet in the past 50 years is majorly driven by human activities.
The industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million in the last 150 years. Human induced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in Earth’s temperatures over the past 50 years.
The decomposition of wastes in landfills, agriculture, ruminant digestion and manure management, synthetic compounds manufacturing, clearing of land for agriculture, industrial activities, and other human activities have increased concentrations of greenhouse gases. The major gases that contribute to the greenhouse effect include Water vapor, Carbon dioxide (CO2), Methane, Nitrous oxide, Chlorofluorocarbons (CFCs).Although, methane is less abundant in atmosphere, it is by far more active greenhouse gas than carbon dioxide.
In many parts of the world, climate change has already caused loss of life, damaging property and affecting livelihoods. The impact of climate change is higher in low income countries, since they have limited capacity to cope with the changes. Some of the consequences of the changing climate include:
- Impacts on human health: The change can cause increased heat related mortality and morbidity, greater frequency of infectious disease epidemics following floods and storms, and substantial health effects following population displacement to escape extreme weather events. Climate change also raises the incidence malaria.
- Impact on water resources: Climate change is leading to melting of snow and glaciers that increases rise in sea level, increase drought and floods, distorts wind flow pattern, decreases water table. More frequent and longer droughts reduce the amount of run-off into rivers, streams and lakes.
- Impact on Agriculture: changes in temperature and rainfall patterns as well as significantly affect agricultural production. Climate change increases physiological stress and fodder quality and availability.
- Impact on Ecosystem: climate change affects the success of species, population, and community adaptation. The rate of climatic warming may exceed the rate of shifts in certain range species, these species could be seriously affected or even disappear because they are unable to resist.
How do our forefathers react to the changing climate? Do we have any traditional (indigenous) mechanism?
Climate change is one of the most complex issues facing us today. So even if we stopped emitting all greenhouse gases today, global warming and climate change will continue as it has natural source of emission. Hence, there has to be response mechanism to reduce the impact of extreme events. There are three major response mechanisms to climate change namely mitigation, adaptation and resilience.
Mitigation and its Strategies
Mitigation measures are those actions that are taken to reduce and control greenhouse gas emissions changing the climate. Moreover, it implies reducing the flow of heat trapping greenhouse gases into the atmosphere, either by reducing sources of these gases or enhancing the “sinks” that accumulate and store these gases(such as the oceans, forests and soil).The goal of mitigations is to avoid significant human interference with the climate system. There are some mitigation measures that can be taken to avoid the increase of pollutant emissions.
- Practice Energy efficiency
- Increase the use of renewable energy such as solar
- Efficient means of transport implementation: electric public transport, bicycle, shared cars etc.
Adaptation and its Strategies
Throughout history, people and societies have adjusted to and coped with changes in climate and extremes with varying degrees of success. Adaptation is simply defined as adapting to life in a changing climate. It involves adjusting to actual or expected future climate. The goal is to reduce our vulnerability to the harmful effects of climate change such as extreme weather events or food insecurity. It also encompasses making the most of any potential beneficial opportunities associated with climate change (for example, longer growing seasons or increased yields in some regions).
Some of the major adaptation strategies include:
- Building flood defenses,
- Plan for heat waves and higher temperatures,
- Installing water-permeable pavements to better deal with floods and storm water
- Improve water storage and use is some of measures taken by cities and towns.
- Landscape restoration and reforestation,
- flexible and diverse cultivation to be prepared for natural catastrophes
- Preventive and precautionary measures (evacuation plans, health issues, etc.)
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