What are climate change and climate variability?
The Intergovernmental Panel on Climate Change (IPCC) defines climate change as “any significant change in climate, such as temperature or precipitation, lasting for an extended period of time, typically decades, whether due to natural variability or as a result of human activity”.
Climate change is primarily a result of global warming, a natural phenomenon. However, human activity has resulted in an increasing amount of greenhouse gas emissions in our atmosphere, which enforce and speed up this occurrence, leading to noticeable shifts in temperature and more unpredictable weather events around the world (see also Section 2 “What are the greenhouse effect and global warming?”).
Climate change has become an internationally recognized problem and its impacts are noted on a global level, throughout many different sectors – agriculture being one of them. The main climate changes resulting from global warming are increasing temperature, changes in rainfall patterns, and the intensity and frequency of extreme events like storms, floods or droughts. These potentially damaging hydro-meteorological events or phenomena are called .
Definition
Climate and weather
Climate is often defined as the weather averaged over a long period of time (normally 30 years).
Weather describes atmospheric conditions at a particular place in terms of air temperature, pressure, humidity, wind speed, cloudiness and precipitation.
Climate hazards or climate stimuli resulting from global warming
- Increase in average global temperature
- Changes in rainfall patterns, i.e. changes in the timing or amount of precipitation (e.g. delayed onset rain, pour distribution, intensity, increasing length and frequency of midseason drought)
- Increase in the frequency or intensity of extreme weather (e.g. storms, floods, cyclones)
- Warming of oceans
- Warming of poles and loss of sea ice resulting in rising sea levels
In addition to global climate change, the phenomenon of climatic variability must be taken into account. Climate variability refers to variations in the current state of the climate, e.g. the amount of rainfall we receive from year to year. Examples of climate variability also include extended droughts, floods, and conditions that result from periodic El Niño and La Niña events (ENSO). While meteorological records show that earth’s temperature is increasing, the analysis of rainfall patterns is more uncertain and shows no clear trends so far. However, it remains variable from season to season.
One economic sector already being affected by climate change is also the most dependent on environmental stability and natural resources: agriculture. However, many of the problems that farmers are facing are not the results of climate change and variability alone. Climate change and variability are rather multipliers of risk, interacting with existing and future hazards to produce unusual situations that might not have been previously experienced.
For example, increased rainfall intensity (a fairly common event) may be said to have caused increased soil erosion. However, the underlying cause is more likely an increased use of herbicides and elimination of shade trees in coffee, resulting in more rapid run-off. The weather is a contributing factor, but not the underlying .
All in all, long-term climate change is expected to lead to more frequent, more extreme or more unpredictable occurrences of climate hazards. This may include the timing, frequency and distribution of rainfall, as well as floods, drought and cyclones.
What are the greenhouse effect and global warming?
The importance of the greenhouse effect
The greenhouse effect is a natural phenomenon that makes life on our planet possible. The term is used in reference to the world and its atmosphere heating up, similar to how a big greenhouse would warm up from the sun. The dominant natural greenhouse gases are H20 (water), CO2 (carbon dioxide), CH4 (methane), and N20 (nitrous oxide).
The earth receives energy from the sun in form of short wave radiation. Solar radiation passes through the atmosphere to reach the earth‘s surface. The earth absorbs some of the energy and radiates the rest back into the atmosphere in the form of infrared radiation. Greenhouse gases (GHGs) block some outgoing long-wave infrared from easily leaving our atmosphere, meaning some heat cannot escape from the atmosphere, back out into space. The GHGs act as a blanket and the atmosphere warms up (see Figure 20).
Without greenhouse gases, or the greenhouse effect, the earth would be a frozen planet, incapable of sustaining life. With no (or just a little) change to the amount of GHGs in the atmosphere, the temperature would remain fairly similar for decades.
Human-made greenhouse effect and global warming
The increase in average global temperatures since the mid-20th century is largely due to the increase in anthropogenic greenhouse gas concentrations, which enforce and speed up global warming. Greenhouse gases are increasingly generated by human activity, such as energy generation, industrial processes, construction or transportation or agricultural activities (see Figure 21).
As GHG emissions continue to increase, the atmosphere will continue to warm. Based on a range of plausible emissions scenarios, average surface temperatures could rise between 1.1°C and 6.4°C by the end of the 21st century (IPCC). The amount of warming depends on the future humans choose, i.e. either reducing GHG emissions to zero impact, limiting them, or continuing with the current rate of warming. Figure 22 shows model simulations by the IPCC of future scenarios for global surface warming up to the year 2100, based on a range of four emissions scenarios.
If GHG emissions were reduced in line with low economic growth (blue line), an increase in global temperatures of just under two degrees Celsius in the next 100 years is expected.
In the case that economic growth remains high (red line), the increase could be as much as four degrees. These numbers may appear small, but their effects will have severe impacts on coffee production (see Chapter 1.2).
Global warming and future climate change predictions
Although the climate is uncertain and will continue to vary from year to year, long-term climate projections suggest that we can generally expect higher average temperatures, both during the day and at night (see Figure 23 for an example of current temperature increase) and a rise in the intensity and frequency of extreme but unpredictable occurrences of meteorological events (climate hazards).
A rising global temperature is expected to provoke more water evaporation and, as a consequence, the water cycle will become more intense with more clouds and rain, especially in tropical areas. While some areas will become wetter, however, others will experience a lack of rain, which will affect crops. It is not only the quantity of water that will change, but also the variability, as some years will be very wet and other years will be very dry. Another problem will be the change in precipitation throughout the year, meaning the rainy season will shift.
Regional warming
In addition to global warming, possible regional warming must be considered. Local warming can be caused by changes in land use and aggravate local climate conditions and extremes. Figure 24 shows an example of how change in local land use results in increasing extremes. Forests can act as a buffer for extreme climates but canopy evaporation cools the local climate and increases the chance of cloud and rain formation. Once the forest is destroyed, a lack of evaporative cooling and reduced precipitation increases temperature and drought. In this case, drought is not caused directly by global climate change, but is very likely made more intense by it.
