Practical Winery
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March/April 2008
There is no convenient "control" planet otherwise similar to Earth that we can leave alone while we are changing the composition of our atmosphere. As a consequence, scientists use climate models, complex computer programs that simulate the three-dimensional movements and thermodynamics of the atmosphere and oceans.
Under development for more than 50 years, the modern generation of models is based on well-understood physics and tested against a large array of observations. While the models are far from perfect, they effectively represent a wide range of features of past and current climate.
When these models are run with only natural contributions driving climate (blue bands in Figure III), the projected global and regional temperatures do not warm over the 20th century, in contrast to the observations (black line in Figure III). But when the models are run with both human and natural contributions, projected temperatures (pink bands in Figure III) and observed temperatures (black lines in Figure III) agree closely.
HOW CAN WE PREDICT WHAT WILL HAPPEN FAR IN THE FUTURE? DIDN'T SCIENTISTS USED TO TALK ABOUT GLOBAL COOLING? All climate models substantially agree about the warming trend currently underway, and project continued and accelerated warming in the future.
All of the approximately 20 major climate models share similar representations of the main forces driving the atmosphere and oceans. They differ in subtle but important details concerning things like the statistical properties of clouds, which are important for determining how much of the sun's energy is reflected back into space before the earth has a chance to absorb it. Most of the ongoing research and discussion on climate models concerns these details.
Climate model projections of global cooling from nearly 40 years ago were also based on a human-caused phenomenon: very small particles (aerosols) from smokestacks and farming causing a cooling effect by reflecting some sunlight back to space.
Climate models have improved in the last 40 years, but the most important reason that the global-cooling scenario did not occur is that emissions of the cooling pollutants decreased (especially through deliberate investments to improve air quality), while the emissions of greenhouse gases have continued to increase exponentially. Aerosols from smokestacks do have a net cooling effect, which has been acting to reduce the warming experienced from greenhouse gases.
The current estimate is that the direct and indirect cooling effects from aerosols may offset about 40% of the warming effect of the main greenhouse gases related to human activity.
WHAT ARE THE PROJECTIONS FOR THE FUTURE? Economic and political decisions about energy and resource use in the coming years will be critical for determining the amount of greenhouse gases emitted. This, in turn, will drive the temperature changes we experience: more emissions will result in greater and more potential damage to the climate change.
Because of inertia in the climate system, we are committed to some warming in the future, even if all greenhouse pollution was stopped today. Experts estimate this locked-in warming from greenhouse gases already in the atmosphere to be between 0.2°C-1°C (0.35°F-1.8°F) (yellow line in Figure IV).
However, this amount of locked-in warming is small compared to the range of warming driven by current and future human emissions.
The range of emissions considered by the IPCC entails a warming more than 66% likely
to fall within the range of +1.1° to 2.8°C (2.0° to 5.0°F) under a lower greenhouse gas emissions scenario, and 2.4° to 6.2°C (4.3° to 11.2°F) under a high emissions scenario. The group's "best estimate" of likely global warming is between +1.6° to 4°C (+3.2° and 7.2°F) by 2100 (Figure IV).
In California, the projected annual average temperature increases for the state over the next century are between approximately +1.7° and 3.0°C (3.0° to 5.5°F) for a lower emissions scenario, and 4.4° to +5.8°C (+7.9° to 10.4°F) for a higher emissions scenario.
This range represents vast differences in the kind of world we will live in.
On the low end of the scale, where the average climate of Monterey today would be more like Sonoma in the future, most regions would have a range of suitable adaptation options, and conditions would be enhanced in some regions. By contrast, the higher end of the range represents a serious to devastating scenario of the future, where Los Angeles is warmer than Death Valley today. Clearly, continuing grapegrowing in such a vastly changed climate would be a serious challenge.
The probability of experiencing extreme climate events, such as very hot days or unseasonal freezes, can be represented by a bell-shaped curve, with the height of the curve representing the likelihood of experiencing the temperatures along the bottom of the curve (Figure V). Most days are somewhere in the middle, with very few days at either extreme.
Projections of climate change leading to increases in average temperatures (represented by the temperature curve shifting to the right in Figure V) helps to visualize why even small shifts in average temperature could mean important changes in impacts of concern at the extremes.
For example, days over a threshold such as 100°F where heat stress can affect vines
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