I came across this question in search results, and I thought I'd give a little summary of possible solutions for this so that future visitors can benefit from it. If you don't want to remove any material from the wood, you can use:
1. Wet towels and an iron.
With this method, the warped wood is exposed to heat while covered with moist towels or blankets.
The towels should be damp but not soaked. The wood should be well covered and placed on a rigid iron bench.
Press a steam iron down onto the end of the wood piece moving systematically to the other end. Repeat the process while observing the results. This method is effective for wood that has mild to moderate warping.
2. Direct sun drying.
This method is ideal in instances when it's hot and the sun is shining.
The warped wood should be well-covered with a wet towel or blanket. The wood should be placed on a straight rigid surface or bench with the inward curve facing downward.
At this point, the wood is put in the sunlight and water is sprinkled on the wrapped towel for good measure. This process may take several days to have an effect. At night, the wood should be stored in a warm environment.
For this method, pressure is applied to the wood on the curved area, and care is taken to avoid damage.
The warped wood should be covered with a wet paper towel on the concave side. Clamp the wood tightly and leave it for about a week while observing the results closely. If the bend is still there, increase the pressure on the curved area.
These are not complete instructions, but rather general summaries of the processes, which are readily found online. For more details, I recommend checking out the guide on Carve Your Creation.
In this case, as others have said, steam bending would likely be the best option.