Brazil's Northeast under the vagaries of the oceans

An IRD researcher and his partners from the State of Ceara's Fundação Cearense de Meteorologia e Recursos Hídricos at Fortaleza studied the impact of climate on production of maize and beans in that State. Their findings are frustrating for the region's farmers: unfavourable conditions lead to very poor yields, yet supposedly "ideal" conditions do not necessarily bring good harvests.

When the oceans get involved

The rainy season in the Nordeste lasts just four months, from February to May. During this time the area receives over 60% of the total annual rainfall. The pattern is linked to the southerly migration of the intertropical convergence zone (1), whose position depends strongly on the sea-surface temperature variability in the tropical Pacific and Atlantic. Any "anomalous" climatic situation over these two oceans around December or January leads to a latitudinal shift of the zone and a disturbance of seasonal rains over the Sertão some weeks later. For example, drought episodes in the Nordeste are often associated with El Niño. Conversely, La Niña can bring on extremely abundant rain and even flooding in this otherwise dry region.

Agricultural vulnerable

Apart from the strong seasonal constraint the region lives under a high inter-annual variability. Some years can be very wet, like 2009, others extremely dry. Agriculture is particularly vulnerable, confronted by this highly erratic climatic situation. Aiming to evaluate the impact of the climate variability on harvest quality and quantity in the Nordeste , the research team first retraced the history of the region's agricultural variables for the second half of the past century. They used agronomic data on the two crops collected by the Instituto Brasileiro de Geografia e Estatistica between 1952 and 2000 and compiled by the Instituto de Planjeamento do Ceará on the planted area, annual production, yield, price and aggregate value (price/yield).

The planted area quadrupled in 50 years, closely in line with the growth of the rural population, rising from 150 000 hectares in the early 1950s to 600 000 ha at the end of the XXth Century. The investigations revealed two distinct periods. The planted area first increased continuously until the end of the 1970s. Then in the early 1980s, the region underwent several consecutive years of drought. Yields and production of maize and beans fell dramatically and never recovered their previous levels in spite of a renewed rise in the 1990s.

Favourable conditions no guarantee of good harvests

The researchers next compared the variations in the harvests with the sea-surface temperature variability of the tropical Pacific and Atlantic. Series of oceanic events that induce severe spells of drought in Brazil's Northeast, in other words an El Niño episode on the Pacific added to abnormally high sea-surface temperatures in the North of the tropical Atlantic and abnormally low ones in the South, are generally associated with very poor harvests. However, the reverse does not necessarily correlate with optimal conditions for agriculture. Paradoxically, "good" conditions -abundant precipitation in the Nordeste , involving a La Niña episode on the Pacific combined with a negative sea-surface temperature anomaly in the tropical North Atlantic and a positive one in the South- do not necessarily produce good harvests. In fact, depending on the distribution of precipitations during the rainy season, the equivalent total quantities of water can have highly variable consequences on the harvests. Thus for example, bacteriological epidemics or insect proliferation are often associated with extremely abundant rainfall. Production is then hardly any higher than normal.

This conclusion highlights the fragility of agriculture in Brazil's Nordeste . The strong intra-seasonal and inter-annual variability in rainfall is only one of the factors which burdens farming in the region. The often extreme poverty of the rural population, a low level of mechanization, the rarity of irrigation systems -which in any case are often inappropriate, the sporadic use of inputs, are some of the factors which restrict crop yields.

Although it evidently will not resolve these structural problems, the research on the relationships between climate and agriculture will at least enable farmers in the North-East to judge better the most appropriate time for sowing, estimate forthcoming yields and as far as possible prevent bad harvests.

Note:

1. The intertropical convergence zone is a belt of a few hundred kilometres wide, where the trade winds converge and low pressures, high cloud cover and strong rainfall occur. This zone varies on either side of the Equator depending on the apparent movement of the sun during the course of the year.