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The Earth's Magnetic Field
Not all scientists agree on the mechanism that creates the magnetic field -- called the magnetosphere -- around the Earth. The general scientific consensus is that it is a product of interactions between the metals at the core of our planet as they spin, producing an electric current, which in turn creates a magnetic field. This process is a dynamic one, constantly changing and routinely going through magnetic reversal, though there appears to be no particular period to this cycle. Because of recently discovered holes in the magnetic field over places like Brazil, and records showing a diminishing field strength over the last century, some scientists speculate that the magnetic poles of the earth may shift within a lifetime.
Diminished Magnetic Field
One possible change in the Earth's magnetic field is that it could diminish substantially. Already, the strength of this field has fallen by almost 10 percent over the last century and shows signs of rapidly accelerating change. Since the magnetosphere protects us from dangerous cosmic rays, a significantly reduced magnetic field would mean, at the very least, increased mutations and cancers in most living things. Additionally, sensitive electronic devices would fail to function reliably, requiring new technological innovations. The worst case scenario -- though extremely unlikely -- would be a total collapse of the protective barrier. If this happened, floods of dangerous radiation could potentially sterilize the world's surface of life and dry up the oceans.
Reversed Magnetic Field
The most likely change to the Earth's magnetic field is that it will rapidly change and reverse polarity. While reversal of the magnetic orientation of the planet would not be catastrophic, it would upset life considerably. Birds, fish and other animals that utilize the Earth's magnetism to navigate would suddenly have to deal with drastic changes. Man would have to alter maps and technologies globally to accommodate the new polarity. Some birds and intelligent mammals would eventually adapt, while lesser species would undergo massive population losses or extinction. Many of the most impacted species are animals that humans utilize for food, meaning humanity would need to offset this loss with other food production means.
Increased Magnetic Field
Though it seems an unlikely possibility, the Earth's magnetic field could increase due to mechanisms scientists do not fully understand. A slightly increased magnetosphere would filter more of the harmful radiation from space, which at first appears beneficial, but on the long term could lead to less diversity in species, which could have negative consequences to evolutionary capability and continued survival. A radically increased magnetic field could also cause the same negative effects of the radiation it filters out -- leading to more tumors, radical mutations in life and interference with electronic and biological navigational systems.