Surface Temperature Anomalies in the River Niger Basin Development Authority Areas, Nigeria ()
1. Introduction
Global warming is occurring worldwide and almost all climate scientists today believe that it is primarily caused by increases in the amount of greenhouse gases in the atmosphere produced by human activities such as burning of fossil fuels and deforestation [1,2]. Global warming is certain as it is now evident from shift in temperature and precipitation patterns, widespread melting of snow and ice, increasing incidents of weather extremes (e.g. Flood, drought, heat wave) and rising mean sea level.
Observations of surface air temperature in weather stations indicate that global mean surface air temperature increased about 0.4˚C since early 1970’s. Parner [3] concluded that the earth’s average surface temperature rose by 0.74 over the period 1960-2005 with a more rapid warming trend over the past 50 years. Also land temperatures have been found to increase about twofold as fast as ocean temperatures: 0.25˚C per decade against 0.13˚C per decade since 1979. Surface temperature has increased by about 0.7˚C since the beginning of Industrial Revolution and this trend is accelerating with average global mean temperature rising at 0.2˚C per decade [3]. Widespread changes in extreme temperatures have been observed over the last 50 years. Current estimates by the Climate Research Unit show 2005 as the second year behind 1998, with 2003 and 2010 as the third warmest year (http://www.cru.uea.ac.uk/cru/info/warming/Retrieved 2011).
However, recent estimates by NASA’s Goddard Institute for Space Studies show that 2005 and 2010 are the warmest year in planets since reliable widespread instrumental measurements became available in the late 19th century exceeding 1998 by a few hundredths of a degree. Although the magnitude of warming varies locally, the warming trend is widespread. Shen [4] observed a cooling trend in the south-western China and Tibetan Plateau and a clear warming trend in North China. Babatolu [5] reported an increase of 0.6˚C in Ondo, Nkemdirim [6] reported that data collected in 1987 showed that warming in excess of 2˚C was as prevalent in the tropics as it was in the temperate latitudes.
A decrease in the diurnal range was identified in the U.S where large area trends showed that maximum temperatures have remained constant or increased only slightly, whereas minimum temperatures have increased at a faster rate [7,8]. The mean temperature in southern China has decreased by 0.8˚C from 1950s to 1980s. It has been observed that temperatures are rising on the African landmass. Colloer [9] observed that Africa is warming faster than the global average and this is likely to continue. Warming has been at an average rate of about 0.5˚C in Africa through the twentieth century with the six warmest years having occurred since 1987, similar to patterns in the rest of the world [10]. From instrumental temperature records, a greater warming trend since the 1960s has been observed although the changes vary from one region to others. Based on the review of a number of studies, Boko [11] observed that there has been a decadal warming rate of 0.29˚C in African tropical forests and 0.10˚C - 0.3˚C in South Africa. He also found that in Ethiopia and South Africa, minimum temperatures have increased slightly faster than maximum or mean temperatures; between 1961 and 2000, there was an increase in the number of warm spells over Southern and Western Africa and a decrease in the number of extremely cold days, while in Eastern Africa there have been some decreasing trends in temperature.
While it is obvious that warming trend is spatially widespread, the magnitude of warming varies locally making empirical studies of climate change at regional and national levels imperative. This study is to fill this gap. Unlike rainfall variability which is well documented in Africa and Nigeria, empirical studies on air temperature trends are very scanty in the country. This study, therefore documents air temperature trends in the Upper and Lower River Niger Basin Development Authority Areas in Nigeria.
2. Materials and Method
2.1. The Study Area
The Upper and Lower River Niger Basin Development Authority Areas (U&LNRBDA) lie mainly between latitudes 7˚N and 12˚N and between longitudes 3˚E and 9˚E in Nigeria (Figure 1). The relief is made up of low pla-
Figure 1. Map of Nigeria showing the Upper and Lower Niger River Basin development authorities.
teau in the northern and eastern part while the southern and western parts are occupied by low hills with an average height of about 600 metres above sea level. These two highlands are separated by the Niger Trough in which older sedimentary rocks were deposited.
U&LNRBDA has wet and dry climates which are respectively associated with prevalence of the moist maritime south-westerly monsoon from the Atlantic Ocean and the dry continental north-easterly wind from the Sahara desert. The sequence of weather types experienced in this region during the course of a given year is determined primarily by the surface position of the Inter Tropical Discontinuity. The data collected for the base period show that the mean rainfall ranges between 1000 m in the north to 130 mm in the north-east while the mean annual temperature ranges between 27˚C in the south to 22˚C in the northeast.
The vegetation of the study area is mainly grassland decreasing in luxuriance northward away from the equator.
2.2. Data Collection
The data base used in this study consists of individual monthly minimum, maximum and annual mean temperatures from 1951-2010 for six stations (Figure 2)
collected from the archives of the Nigeria Meteorological Agency, Oshodi. The stations selected are those that possess complete data set for the base period and also show adequate spatial distribution.
2.3. Data Analysis
Each of the station mean, annual mean, annual minimum and mean annual maximum temperature series were analyzed for fluctuation using Standardized Anomaly Index (SAI) which is a commonly used index for regional climate change studies. Station temperature is expressed as a standardized departure 
from the longterm mean (i.e. the mean of the base period), calculated as:
where r is the mean temperature in the year, ri is the long-term mean, and σ is the standard deviation of annual mean temperature for the long-term mean. For a given region or zone, area integrated temperature R is calculated as
where I is the number of stations and summation is made over all I stations. Thus, a value of I indicates an average
Figure 2. Map of the Upper and Lower Niger River Basin development authorities showing climate data collection stations.
regional anomaly of one standard deviation from the mean.
The regional temperature series were also investigated for trends using Spearman’s rank correlation coefficient.
3. Result and Discussion
The temperature anomalies which occur in the U& LNRBDA during 1951-2010 periods are described for the mean annual, mean annual maximum and minimum temperatures. Figure 3 shows two distinct patterns of temperature anomalies-a period when below long-term average was dominated (cooling) and a period when above long-term average was most persistent (warming). For mean annual temperature, the period 1951-66 was marked with below long-term average indicating a period of cooling. The remarkable thing about this period is that the cooling which start from 1951 continued with no respite till 1966. In contrast, above average mean annual temperature was observed for the period 1967-2007. Temperatures increased steadily with few respites and from 1996 above average temperatures were very consistent till 2007. During this 45-year period only seven years have below average temperature.
The anomalies in the mean annual maximum temperature followed the patterns of the mean annual temperature anomalies (Figure 3(b)), below long-term average maximum temperatures persisted during the period spanning 1951-70. This period has only three years of above average temperature (1957, 1961 and 1967). There is preponderance of warm years in the period 1971-2007 with only six years recording below average temperatures. Most of the years of this period experienced strong positive departures.
Figure 3(c) shows that it is short-term of below and above average temperature conditions of 2 - 5 years that characterized the mean annual minimum temperature